Gun held by officer if legal (5)
I believe the answer is:
rifle
'gun' is the definition.
(I know that rifle is a type of firearm)
'held by officer if legal' is the wordplay.
'held by' indicates a hidden word.
'RIFLE' is hidden within 'officer if legal'.
(Other definitions for rifle that I've seen before include "Strip - firearm" , "Gun - ransack" , "One that has stock" , "Gun; thoroughly search" , "Root" .)