Weapon — mine is hollow (6)
I believe the answer is:
armpit
'is hollow' is the definition.
(I've seen this before)
'weapon mine' is the wordplay.
'weapon' becomes 'arm' (synonyms).
'mine' becomes 'pit' (as in a mining pit).
'arm'+'pit'='ARMPIT'
(Other definitions for armpit that I've seen before include "Hollow between limb and shoulder" , "Depression under an upper limb" , "Bodily hollow" , "Leave" , "evident given "hands up"!" .)