Wrong part for a brass instrument (3)
I believe the answer is:
sin
'wrong' is the definition.
(thesaurus)
'part for a brass instrument' is the wordplay.
'part' indicates the answer is hidden within the clue.
'SIN' is hidden within 'for brass instrument'.
(Other definitions for sin that I've seen before include "Trespass" , "trip" , "Transgress" , "Transgression of moral law" , "Pride, say" .)