Gun held by farmer if legal (5)
I believe the answer is:
rifle
'gun' is the definition.
(I know that rifle is a type of firearm)
'held by farmer if legal' is the wordplay.
'held by' indicates the answer is hidden within the clue.
'RIFLE' can be found hidden inside 'farmer if legal'.
(Other definitions for rifle that I've seen before include "Shoulder weapon" , "Pillage; a weapon" , "Search and rob - with long gun?" , "Gun - steal" , "strip bare?" .)