Plot is involving gun (6)
I believe the answer is:
pistol
'gun' is the definition.
(pistol is a kind of gun)
'plot is involving' is the wordplay.
'involving' indicates an anagram.
'plot'+'is'='plotis'
'plotis' is an anagram of 'PISTOL'.
(Other definitions for pistol that I've seen before include "Hand weapon" , "Rectangular" , "Small gun" , "Officer's side-arm" , "Hand-held firearm" .)