Old bird, occasionally immoral (3)
I believe the answer is:
moa
'old bird' is the definition.
(extinct bird)
'occasionally immoral' is the wordplay.
'occasionally' indicates alternate letters (letters taken at regular intervals or occasions).
The alternate letters of 'immoral' are 'MOA'.
(Other definitions for moa that I've seen before include "Extinct flightless bird of New Zealand" , "Bird's gone for ever" , "Extinct large flightless NZ bird" , "Old bird" , "bird that became extinct" .)