You are able to cast the cleric spells at a higher level.
While spells known / prepared are handled on a class basis, your spell slots are combined.
PHB, page 164, Multiclassing, Spellcasting, Spell slots
Spell Slots. You determine your available spell slots
by adding together all your levels ... Use this total to determine your spell slots by consulting the Multiclass Spellcaster table.
Regarding your ability to use the cleric spells in these slots, a few sentences later it says
If a lower-level spell that you cast, like burning hands, has an
enhanced effect when cast using a higher-level slot, you
can use the enhanced effect.
Consult the multiclass spell slot table on the next page, and those are your slots. You can use any of the spells you have prepared, from either class, in any of those slots.
So, for example, as Sorcerer 19 / Cleric 1 you could cast Cure Wounds in your sole 9th level spot, healing for 9d8 + Wis, but then you would no longer be able to use that slot for sorcerer spells.