Catches back of gun (4)
I believe the answer is:
sten
'gun' is the definition.
(British sub-machine gun)
'catches back' is the wordplay.
'catches' becomes 'nets' (netting is a kind of catching).
'back' says the letters should be written backwards.
'nets' back-to-front is 'STEN'.
'of' acts as a link.
(Other definitions for sten that I've seen before include "Type of machinegun" , "dart of pain to Rob Roy, say" , "weaponry" , "One may be trained" , "Light sub-machine (gun)" .)