The new Cartomancer feat from The Book of Many Things has a feature, Hidden Ace, that says "when you finish a long rest, you can choose one spell from your class's spell list and imbue that spell into a card." After this you can "flourish the card and cast the spell within."
A multiclassed Cleric 1 / Wizard 10, who has access to sixth-level spell slots, could not normally prepare Heal, because "You determine what spells you know and can prepare for each class individually, as if you were a single-classed member of that class" (Player's Handbook, Ch. 6).
However, Hidden Ace doesn't require the imbued spell to be prepared, and doesn't seem to include a similar multiclassing restriction. Does that mean the same multiclassed Cleric 1 / Wizard 10 with the Cartomancer feat could imbue a sixth-level spell that is on the Cleric spell list but not the Wizard spell list, such as Heal, into their card?
Best Answer
Yes, you can
The closest equivalent I can think of is the Mizzium Apparatus from Guildmaster's Guide to Ravnica. That allows you to
It's also the most broken Uncommon item I've ever seen.
The relevant thing is, that item definitely allows you to cast spells for which you have slots, but you couldn't prepare. There's a build that multiclasses into three or four full spellcasters, to basically get access to each class' spells as if you were a single-classed member of that class.
So RAW, yes, you can use Hidden Ace to cast Heal if you are a 1st level cleric. Whether or not your DM should allow it depends on your play group and style. Unlike the item, Hidden Ace only gives you one spell one time per day, so it's probably not quite as broken.