Spoil backing of legato bassoon piece (8)
I believe the answer is:
sabotage
'spoil' is the definition.
(thesaurus)
'backing of legato bassoon piece' is the wordplay.
'backing of' shows that the letters should be reversed in order.
'piece' indicates the answer is hidden within the clue.
'egatobas' is hidden in the letters of 'legato bassoon'.
'egatobas' back-to-front is 'SABOTAGE'.
(Other definitions for sabotage that I've seen before include "spoil things" , "Treachery!" , "Deliberate wrecking" , "Deliberately destroy" , "(E.g. factory) wrecking" .)