Box holding earl's weapon (5)
I believe the answer is:
spear
'weapon' is the definition.
(spear is a kind of weapon)
'box holding earl's' is the wordplay.
'box' becomes 'spar' (sparring is a kind of boxing).
'holding' is an insertion indicator.
'earl' becomes 'e'.
'spar' placed around 'e' is 'SPEAR'.
(Other definitions for spear that I've seen before include "Weapon to throw" , "asparagus for example" , "Asparagus shoot" , "Long pointed weapon" , "pierce with this?" .)