Nominally favourite concert piece (5)
I believe the answer is:
peter
'piece' is the definition.
(peter is a kind of piece)
'nominally favourite concert' is the wordplay.
I cannot really see how this works, but
'favourite' could be 'pet' (synonyms) and 'pet' is present in the answer.
The remaining letters 'er' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for peter that I've seen before include "- Grimes (Britten)" , "Lord -- Wimsey (Sayers)" , "Boy's name - safe" , "and 4 across Doctor Who actor" , "' - - Pan' by Barrie" .)