I've asked before, but no one has answered: what assassin ability in the 5e play test materials is stopping a character from changing directions and becoming a cleric? Where specifically does the problem lie?
I think it's less precise assassin abilities stopping a change in direction, and more to do with the ability score requirements. If someone wants to take a level in order to badly cast a cleric spell every now and then, I say let him.
Maybe feats could fill that niche better than full multi-classing?
But what if he wants to go further by get better at being a cleric by virtue of experience (class level) instead of raw ability (ability score).