Box carried by his partner (4)
I believe the answer is:
spar
'box' is the definition.
(spar can mean to box)
'carried by his partner' is the wordplay.
'carried by' says the answer is hidden in the clue.
'SPAR' is hidden within 'his partner'.
(Other definitions for spar that I've seen before include "Box (lightly) - fight (verbally)" , "Stout pole to support rigging" , "Pole used to support sails or rigging" , "Box - girder" , "Raps with wooden beam" .)