Singer in first ENO Rigoletto (5)
I believe the answer is:
tenor
'singer' is the definition.
(tenor is a kind of singer)
'in first eno rigoletto' is the wordplay.
'in' says the answer is hidden in the clue.
'TENOR' is hidden in the letters of 'first eno rigoletto'.
(Other definitions for tenor that I've seen before include "One singing" , "Vocal range" , "Singer (one of three?)" , "Meaning - male singing voice" , "Big bell" .)