Joined English National Opera on retirement (3)
I believe the answer is:
one
'joined' is the definition.
(I've seen this in another clue)
'english national opera on retirement' is the wordplay.
'english national opera' becomes 'ENO' (abbreviation).
'on retirement' says the letters should be written in reverse (retire can mean to go back).
'eno' reversed gives 'ONE'.
(Other definitions for one that I've seen before include "This is singular" , "Single and undivided" , "Single, sole" , "A certain; single person" , "Unit" .)