Plot is involving firearm (6)
I believe the answer is:
pistol
'firearm' 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 "Piece" , "Hand weapon" , "Shakespearean - weapon" , "potential killer?" , "pop" .)