Prize belly on sailor (7)
I believe the answer is:
jackpot
'prize' is the definition.
(I know that jackpot is a type of prize)
'belly on sailor' is the wordplay.
'belly' becomes 'pot' (as in a pot belly).
'on' says to put letters next to each other.
'sailor' becomes 'jack' (informal term for a sailor).
'pot' put after 'jack' is 'JACKPOT'.
(Other definitions for jackpot that I've seen before include "pool" , "First prize" , "Great thing to hit?" , "Huge gambling prize" , "Kitty -- top prize" .)