Plot is about a weapon (6)
I believe the answer is:
pistol
'a weapon' is the definition.
(pistol is a kind of weapon)
'plot is about' is the wordplay.
'about' indicates anagramming the letters.
'plot'+'is'='plotis'
'plotis' with letters rearranged gives 'PISTOL'.
(Other definitions for pistol that I've seen before include "Shakespearean - weapon" , "Shakespearean character" , "Spoilt these guns" , "Hand weapon" , "A small hand-gun" .)