Box contains energy weapon (5)
I believe the answer is:
spear
'weapon' is the definition.
(spear is a kind of weapon)
'box contains energy' is the wordplay.
'box' becomes 'spar' (sparring is a kind of boxing).
'contains' is an insertion indicator.
'energy' becomes 'e' (physics symbol as in E=mc2).
'spar' enclosing 'e' is 'SPEAR'.
(Other definitions for spear that I've seen before include "stick" , "A bit of asparagus" , "Weapon to throw" , "Pole with pointed end" , "Thrown weapon" .)