Drink ruined a grocer at sea — he does have loads on his mind (10)
I believe the answer is:
supercargo
'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.
'a'+'grocer'='agrocer'
'agrocer' is an anagram of 'ercargo'.
'sup'+'ercargo'='SUPERCARGO'
(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" .)