Plot is involving a gun (6)
I believe the answer is:
pistol
'a gun' is the definition.
(I know that pistol is a type of firearm)
'plot is involving' is the wordplay.
'involving' indicates an anagram.
'plot'+'is'='plotis'
'plotis' with letters rearranged gives 'PISTOL'.
(Other definitions for pistol that I've seen before include "Hand weapon" , "making an appearance in Shakespeare's King Henry V" , "Officer's side-arm" , "Piece" , "Shakespearean character" .)