Assist with unfinished opera (3)
I believe the answer is:
aid
'assist' is the definition.
(synonyms)
'unfinished opera' is the wordplay.
'unfinished' means to remove the last letter.
'opera' becomes 'aida' (Verdi opera).
'aida' with its last letter taken off is 'AID'.
'with' is the link.
(Other definitions for aid that I've seen before include "Help or succour" , "Good point" , "grant" , "Auxiliary" , "charity" .)