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' indicates the answer is hidden within the clue.
'SPAR' can be found hidden inside '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" .)