Tar over grass, constructing pool (7)
I believe the answer is:
jackpot
'pool' is the definition.
(pool of winnings or prize money)
'tar over grass' is the wordplay.
'tar' becomes 'jack' (both can mean a sailor).
'over' says to put letters next to each other.
'grass' becomes 'pot' (cannabis).
'jack'+'pot'='JACKPOT'
'constructing' acts as a link.
I am not very happy about this link. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for jackpot that I've seen before include "Great thing to hit?" , "Accumulated prize" , "Big win from one-arm bandit" , "best prize!" , "major win" .)