It's a lot of money for a tar vessel (7)
I believe the answer is:
jackpot
'it's a lot of money' is the definition.
(I've seen this before)
'tar vessel' is the wordplay.
'tar' becomes 'jack' (both can mean a sailor).
'vessel' becomes 'pot' (type of container).
'jack'+'pot'='JACKPOT'
'for a' is the link.
(Other definitions for jackpot that I've seen before include "more than just change for the better" , "Big win from one-arm bandit" , "The big pay-out - hit it" , "lot of money" , "First prize" .)