You are merging together what you can cast, and what spell slots you have.
Because you are multi-class full spell casters, the number and the level of spell slots you have is not diminished in any way.
But when it comes to the level of spell a class can cast at, that is determined by the level of that single class.
So your first level cleric can only cast 1st-level spells (Bless, Cure Wounds, etc). But they can cast those 1st-level spells using a 5th-level spell slot for a biggest affect.
So if the scroll is for a spell that exists on the Cleric spell list, but not the Druid, then anything above 1st-level is higher than you can cast. If it exists on the Druid spell list, then it’s fine.
Just have to keep remembering that spell level is not the same as spell level slot.