Weapon traps set up (4)
I believe the answer is:
sten
'weapon' is the definition.
(I've seen this before)
'traps set up' is the wordplay.
'traps' becomes 'nets' (net is a kind of trap).
'set up' is a reversal indicator (in a down clue, letters go up).
'nets' backwards is 'STEN'.
(Other definitions for sten that I've seen before include "Light sub-machine (gun)" , "Light portable submachine-gun" , "dart of pain to Rob Roy, say" , "Jock's bound" , "Type of light WWII machine gun" .)