Big prize a sailor has to shoot for (7)
I believe the answer is:
jackpot
'big prize' is the definition.
(I've seen this before)
'a sailor has to shoot for' is the wordplay.
'a sailor' becomes 'jack' (informal term for a sailor).
'has' is a charade indicator (letters next to each other).
'to shoot for' becomes 'pot' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'jack'+'pot'='JACKPOT'
Can you help me to learn more?
(Other definitions for jackpot that I've seen before include "Huge gambling prize" , "more than just change for the better" , "The big pay-out - hit it" , "major win" , "Big win from one-arm bandit" .)