It's a "one-time" use. You just take a move from another playbook, instead of a move you could normally take.
I'll try to base this interpretation with rules texts.
As we know, not every move has a Trigger. This is how many moves look like:
When you (do X; trigger), roll +Stat. On a 7-9, (Y happens). On a 10+, (Z happens).
But some moves look like this:
For example, the Armored move of many classes, or moves like this:
Choose an additional appetite.
Scent of Blood
When you hack and slash an enemy, your next attack against that same foe deals +1d4 damage.
So the thing to learn here is that not every move is something that you can trigger. (You used the term, "use". In fact, you don't use moves, ever. You describe an action, and if it triggers a move you follow through that move.) Sometimes a move is.
So what does the Multiclass (Dabbler/Initiate/Master) move say?
Get one move from another class. Treat your level as one lower for choosing the move.
Do what it says. You take one move from another class. It makes no mention about how or when you can change that one move. There is no trigger. You cannot trigger this move.
Is the multiclass move a one time use, or can you use the move repeatedly and swap out the skill that you take from another class?
Is the key element here.
You cannot use a move.
You cannot use a Multiclass Dabbler move. You cannot trigger it, either, because there is no trigger.
More support for this interpretation
Under Playing the Game, Character Change:
The multiclass moves allow you to gain moves from another class. You get to choose any move of your level or lower. For the purpose of multiclassing, any starting class moves that depend on each other count as one move—the wizard’s cast a spell, spellbook, and prepare spells for example. If a move from another class refers to your level, count your levels from the level where you first gained a move from that class.
This gives the idea that you only get to choose a move, one move.
The example under Moves, Multiclass Moves:
When Ajax gains 3rd level he takes Multiclass Dabbler to get Commune and Cast a Spell from the Cleric class. He casts and prepares spells like a first level Cleric: first level spells and rotes only, a total of 2 levels of spells prepared. When he later gains 4th level, he prepares and casts spells as a second level Cleric.
If Ajax could choose to take other moves as well, the example might be lengthier about how changing a move to another works when there are spells prepared, and how it affects Spells/Commune.