Is this fine as a hybrid?

Um, possibly.

The bigger problem is that it doesn't work. Your creatures will only get put into graveyards during resolution of spells (for instance Doom Blade) or as a result of State Based Actions (due to lethal damage, Deathtouch, zero toughness). Neither of those is a time you can cast spells.

The simplest solution I can think of is to give the card no mana cost (making it uncastable normally), and the ability "Whenever a zombie or cleric you control is put into a graveyard from the battlefield, if ~ is in your hand, you may pay . If you do, cast ~ without paying its mana cost."

I think it'd work fine as a 7-mana instant, phrased as "The next time a Zombie or Cleric you control is put into a graveyard from the battlefield this turn, ..." It's very slightly different in effect, but only in the same way that regeneration and damage-prevention changed with the 6th Edition rules from this kind of template to modern wording. (See the reminder text on Regeneration and the way Kitsune Healer is phrased.)

That's true, I suppose. The major difference is that your (much simpler!) version may let the opponent negate the spell by somehow protecting the creature that's about to die. However, that's probably pretty unlikely.

Incidentally, it's just about fine as a W-B hybrid for the reasons Alex gives on Daring Curiousity - you're dropping colour-aligned types. It's a little dubious for White to get free zombies, but I think the cleric clause can make it palatable.

There's only two things that make this from being strictly worse than Tooth and Nail:
1). It's a Black-White Hybrid Spell, not Green, and
2). It's an instant.
I would think there are enough restrictions here that the casting cost can be brought down by at least . Also, I'd suggest putting the creatures into play... unless casting spells is important to your set. Seems weird that a Remove Soul can stop this spell from being effective.