Weapon made sir nasty (7)
I believe the answer is:
sidearm
'weapon' is the definition.
(I've seen this before)
'made sir nasty' is the wordplay.
'nasty' is an anagram indicator.
'made'+'sir'='madesir'
'madesir' with letters rearranged gives 'SIDEARM'.
(Other definitions for sidearm that I've seen before include "Officer may carry this" , "weapon" , "One may be fired" , "Piece" .)