Time after Post Office returned major work? (5)
I believe the answer is:
opera
'major work?' is the definition.
(operas are musical works)
'time after post office returned' is the wordplay.
'time' becomes 'era' (era is a kind of time).
'after' says to put letters next to each other.
'post office' becomes 'po' (abbreviation).
'returned' shows that the letters should be reversed in order.
'po' reversed gives 'op'.
'era' after 'op' is 'OPERA'.
(Other definitions for opera that I've seen before include "Tosca, for example" , "Music and drama production" , "Macbeth, say" , "Norma is one" , "Eg, La forza del destino" .)