Pole in box (4)
I believe the answer is:
spar
This clue is a double definition.
'pole' is the first definition.
(I know that spar is a type of pole)
'box' is the second definition.
(spar can mean to box)
'in' is the link.
(Other definitions for spar that I've seen before include "Practice boxing with wooden beam" , "Practise boxing with a pole?" , "Fight (in training) - pole (on boat)" , "Mineral; plane's wing beam" , "Fight - something probably rigged?" .)