Put up gun catches (4)
I believe the answer is:
nets
'catches' is the definition.
(netting is a kind of catching)
'put up gun' is the wordplay.
'put up' says the letters should be written backwards.
'gun' becomes 'sten' (British sub-machine gun).
'sten' written backwards gives 'NETS'.
(Other definitions for nets that I've seen before include "Traps, to catch fish say" , "practice area" , "puts ball into goal" , "Catches (fish)" , "Practice pitches (cricket)" .)