Maybe blackens Peter in opera (6)
I believe the answer is:
grimes
This clue is a double definition.
'maybe blackens' is the first definition.
(grime can mean to blacken with grime)
'peter in opera' is the second definition.
(Peter Grimes is an opera by Benjamin Britten)
(Other definitions for grimes that I've seen before include "Peter - - (Britten opera)" , "gets dirty" , "Singing fisherman" , "Peter the Fisherman" .)