Secure a gun after start of fight (6)
I believe the answer is:
fasten
'secure' is the definition.
('fasten' can be a synonym of 'secure')
'a gun after start of fight' is the wordplay.
'gun' becomes 'sten' (British sub-machine gun).
'after' means one lot of letters go next to another.
'start of' says to take the initial letters.
The first letter of 'fight' is 'f'.
'a'+'sten'='asten'
'asten' after 'f' is 'FASTEN'.
(Other definitions for fasten that I've seen before include "Fix securely" , "Shut" , "Nail" , "Close, make secure" , "Fix -- make tight" .)