Box containing Eastern weapon (5)
I believe the answer is:
spear
'weapon' is the definition.
(spear is a kind of weapon)
'box containing eastern' is the wordplay.
'box' becomes 'spar' (sparring is a kind of boxing).
'containing' indicates putting letters inside.
'eastern' becomes 'e'.
'spar' going around 'e' is 'SPEAR'.
(Other definitions for spear that I've seen before include "Long pointed weapon" , "Shafted weapon for throwing or thrusting" , "Pole with pointed end" , "Thrown weapon" , "Primitive weapon" .)