Plot is to involve gun (6)
I believe the answer is:
pistol
'gun' is the definition.
(I know that pistol is a type of gun)
'plot is to involve' is the wordplay.
'to involve' indicates anagramming the letters.
'plot'+'is'='plotis'
'plotis' is an anagram of 'PISTOL'.
(Other definitions for pistol that I've seen before include "pop" , "Type of gun" , "Spoilt these guns" , "Hand-held firearm" , "A small hand-gun" .)