Drink ruined a grocer at sea — he does have loads on his mind (10)
I believe the answer is:
'at sea he does have loads on his mind' is the definition.
(someone who looks after bits of cargo)
'drink ruined a grocer' is the wordplay.
'drink' becomes 'sup' (to sup is to drink).
'ruined' indicates an anagram.
'agrocer' is an anagram of 'ercargo'.
(Other definitions for supercargo that I've seen before include "Ship's officer" , "Merchant navy officer responsible for what a ship is carrying" , "Owner's representative on merchant ship" .)