Box containing eastern throwing weapon (5)
I believe the answer is:
spear
'throwing weapon' is the definition.
(I've seen this before)
'box containing eastern' is the wordplay.
'box' becomes 'spar' (sparring is a kind of boxing).
'containing' means one lot of letters goes inside another.
'eastern' becomes 'e'.
'spar' enclosing 'e' is 'SPEAR'.
(Other definitions for spear that I've seen before include "Javelin or harpoon" , "Weapon; asparagus stem" , "mint may be added" , "Long pointed weapon" , "Simple weapon" .)