"You're on!"

DEAL

A handshake might seal this four-letter agreement before a game begins.

"You're on!" is a casual way to agree to something, like sealing a bet or accepting a challenge. The answer "DEAL" fits because it’s what you’d say to confirm an agreement—think of poker ("You’re on? Deal the cards!") or making a handshake deal. The clue plays on the phrase’s double meaning: both as encouragement ("your turn!") and as shorthand for striking a bargain. It’s a straightforward pun where the answer sounds like the natural response.