Box concealing ultimate in offensive weapon (5)
I believe the answer is:
spear
'weapon' is the definition.
(spear is a kind of weapon)
'box concealing ultimate in offensive' is the wordplay.
'box' becomes 'spar' (sparring is a kind of boxing).
'concealing' is an insertion indicator.
'ultimate in' indicates one should take the final letters.
The final letter of 'offensive' is 'e'.
'spar' going around 'e' is 'SPEAR'.
(Other definitions for spear that I've seen before include "Sharp weapon" , "Sharp-tipped weapon" , "Spike" , "Asparagus shoot" , "stick" .)