Big pool a sailor has to top up! (7)
I believe the answer is:
jackpot
'big pool' is the definition.
'jackpot' can be an answer for 'pool' (pool of winnings or prize money). I am unsure of the 'big' bit.
'a sailor has to top up' is the wordplay.
'a sailor' becomes 'jack' (informal term for a sailor).
'has' says to put letters next to each other.
'to top up' becomes 'pot' (I am not sure about this - if you are sure you should believe this answer much more).
'jack'+'pot'='JACKPOT'
Can you help me to learn more?
(Other definitions for jackpot that I've seen before include "pool" , "Accumulated prize" , "Great thing to hit?" , "First prize" , "Large cash prize" .)