Strict Republican covered by gun (5)
I believe the answer is:
stern
'strict' is the definition.
('stern' can be a synonym of 'strict')
'republican covered by gun' is the wordplay.
'republican' becomes 'r' (abbreviation).
'covered by' means one lot of letters goes inside another.
'gun' becomes 'sten' (British sub-machine gun).
'r' going into 'sten' is 'STERN'.
(Other definitions for stern that I've seen before include "Stiff - back - not smiling" , "Strict, grimly serious" , "Poop" , "Where sailors may be" , "Severe part of the ship?" .)