Muse employed by some singer at opera (5)
I believe the answer is:
erato
'muse' is the definition.
(I know that Erato is a muse)
'employed by some singer at opera' is the wordplay.
'employed by' indicates the answer is hidden within the clue.
'ERATO' is hidden within 'some singer at opera'.
(Other definitions for erato that I've seen before include "Greek muse - orate (anag)" , "goddess" , "Arts patron" , "Greek muse of lyric and love poetry" , "Inspiring female" .)