Forbidding right to wear gun (5)
I believe the answer is:
stern
'forbidding' is the definition.
('stern' can be a synonym of 'forbidding')
'right to wear gun' is the wordplay.
'right' becomes 'r' (common abbreviation).
'to wear' indicates putting letters inside.
'gun' becomes 'sten' (British sub-machine gun).
'r' going into 'sten' is 'STERN'.
(Other definitions for stern that I've seen before include "Where sailors may be" , "Unsmiling" , "Grim, strict" , "End part" , "Rigorous - back of boat" .)