Fierce Retribution

Instant

Cleave (You may cast this spell for its cleave cost. If you do, remove the words in square brackets.)
Destroy target [attacking] creature.

2021-11-19:
If you cast a spell for its cleave cost, that spell doesn't have any of the text in square brackets while it's on the stack.
2021-11-19:
A cleave cost is an alternative cost that's paid instead of the spell's mana cost. Casting a spell for its cleave cost doesn't change the spell's mana value.
2021-11-19:
You can't cast a spell for both its cleave cost and another alternative cost. For example, if an effect gives an Alchemist's Retrieval in your graveyard a flashback cost of {U}, you can't cast it from your graveyard for its cleave cost.
2021-11-19:
If an effect allows you to “cast a spell without paying its mana cost,” you can't cast that spell for its cleave cost.