Weapon plot is badly organised (6)
I believe the answer is:
pistol
'weapon' is the definition.
(pistol is a kind of weapon)
'plot is badly organised' is the wordplay.
'badly organised' indicates anagramming the letters.
'plot'+'is'='plotis'
'plotis' anagrammed gives 'PISTOL'.
(Other definitions for pistol that I've seen before include "Officer's side-arm" , "Piece" , "A small hand-gun" , "Shakespearean - weapon" , "Rectangular" .)