The Light Domain offers a way to make the ally unseen or unseeable
Since you allow spells of up to 5th level, the Light Domain has as a domain spell wall of fire. If you create a wall of fire around your ally - make sure the fire damage is on the outside and your ally is on the inside of a circle or a wall - here is what happens:
a. Anyone who tries to pass through it takes 5d8 fire damage
b. Nobody can see your ally. Why? The wall is opaque.
It is 4th level spell, and meets your other criteria of lasting for one combat:
DURATION : Concentration 1 Minute
You create ...{snip} a ringed wall up to 20 feet in diameter, 20 feet high, and 1 foot thick. The wall is opaque and lasts for the duration. > {snip} One
side of the wall, selected by you when you cast this spell, deals 5d8
fire damage to each creature that ends its turn within 10 feet of that
side or inside the wall. A creature takes the same damage when it
enters the wall for the first time on a turn or ends its turn there.
The other side of the wall deals no damage.
The challenge for the cleric-spellcaster is to position that "ring of fire" around your ally such that your ally is not in the burning part of the wall of fire. Tactically, I'll suggest at least a 5' space between the ally and the wall's inner face. They are inside of it, no damage done to the ally, and anyone within 10' of the other side, or "in/on" the other side, takes fire damage.
And, they can't see your ally since the wall is opaque.
Plus, the visual effect is neat: the ally falls and up springs a wall of fire around them! If your ally is named Brunhild, so much the better! 👍
Another spell option is banishment (also a 4th level spell) which displaces the ally to some other plane of existence for the duration of the combat (one minute). I generally use it to take enemies out of a fight, but you can also take your ally out of the fight. The enemy can't see your ally, who is now on another plane. You and the rest of your party are the enemy's immediate problem ... but if your ally makes their Charisma saving throw, this one blows up in your face with a 4th level spell slot burned.
Caveat
Either of the above is a concentration spell, so if you take damage this may fail if your concentration save fails. Roll well, Grasshopper. The invisibility spell has a similar limitation, since it too is a concentration spell.
Minor Illusion ...
... may work, if you have it from a race or a feat. If you are a Gnome cleric or a High Elf cleric, and you have the minor illusion cantrip, you may be able to cast an illusion between the enemy and your ally such that it blocks their line of sight. This one is very situational in its effectiveness, since you have not provided a battle map / battlefield for me to assess. In some cases this will create a visual obstruction (pile of bones 5' high, a wall 5' high, pile of logs 5' high, etc.). In other cases that will not work so well. The caveat on this cantrip is - if the enemy takes an Action to investigate this illusion, or tries to physically interact with it, the illusion will not fool them. This is a lower odds bet in Vegas, but it might work.
Your two best bets are wall of fire (light domain cleric) and banishment (any cleric).
Trickery Domain and using your duplicate to stand in front of the fallen ally.
This one's also tactically situational.
At second level, if you are a Trickery Cleric, you can create a perfect duplicate of yourself. It stands where you want it to stand. Having it stand between your ally and the enemy will be more effective if there is only one enemy, and it offers at least partial cover - but it's probably less effective if there's a small horde involved. Effective in some tactical situations, and maybe the kind of misdirection that you are looking for. If one is on a spell level budget, this may be the best option, but I'll take Wall of Fire if I can get it.
How many enemies are there? Blindness is an option.
You can blind up to 4 enemies if using a level 5 casting of Blindness. (At level 2 you get 1, at level 3 you get 2, at level 4 you get 3). While it does not make your ally hidden, the enemy can't see your ally because they are blinded. But, the enemy gets a saving throw, so this may simply fail. That's a similar risk to the risk of using banishment.
Please note @Kirt's answer for some other good options (his suggestion of Stone Shape is innovative if there is stone available ... ).
Now, if you have a ring of spell storing ...
... and you have pre-loaded an invisibility spell, then you (or anyone else in the party who is attuned) can make your ally invisible. This is an edge case since magic items of this quality cannot be assumed.