Gun put back outside first space for retreats (5)
I believe the answer is:
nests
'for retreats' is the definition.
(nest is a kind of retreat)
'gun put back outside first space' is the wordplay.
'gun' becomes 'sten' (British sub-machine gun).
'put back' is a reversal indicator.
'outside' means one lot of letters goes inside another.
'first space' becomes 's' (1st letter of 'space').
'sten' back-to-front is 'nets'.
'nets' going around 's' is 'NESTS'.
(Other definitions for nests that I've seen before include "Cosy secluded retreats" , "Bird houses" , "Sets of small tables that fit closely together" , "Structures for laying eggs" , "Lodges" .)