To accommodate the gun, did expand (7)
I believe the answer is:
distend
'expand' is the definition.
(distending is a kind of expanding)
'to accommodate the gun did' is the wordplay.
'to accommodate' means one lot of letters goes inside another.
'the gun' becomes 'sten' (British sub-machine gun).
'sten' put within 'did' is 'DISTEND'.
(Other definitions for distend that I've seen before include "Stretch" , "Increase" , "Swell with internal pressure" , "Cause to swell" , "balloon" .)