Can Riposte be played in response to Cleave during a showdown after Cleave has already resolved?

Ruling: Riposte cannot be played after Cleave has resolved. Riposte requires both a friendly unit and a spell on the chain as targets to be played legally. Once Cleave resolves, there is no spell on the chain anymore, making it too late to play Riposte.

Sequence:

• Cleave is played and opens a Chain
• Both players have the opportunity to play Reactions while Cleave is on the chain
• Once Cleave resolves, Riposte can no longer be played because there is no spell on the chain to target

Nuances:

• Units and Gears don't provide Reaction windows as they fall right off the chain
• Riposte specifically requires both a friendly unit AND a spell on the chain to be played legally