Operetta season on after April (7)
I believe the answer is:
maytime
'april' is the definition.
Both the definition and answer are singular nouns.
Perhaps you can see an association between them that I can't see?
'operetta season on' is the wordplay.
'operetta' becomes 'may' (I am not sure about this - if you are sure you should believe this answer much more).
'season' becomes 'time' (both can mean a period).
'on' means one lot of letters go next to another.
'may'+'time'='MAYTIME'
'after' acts as a link.
Can you help me to learn more?
(Another definition for maytime that I've seen is " When initially you should see socialist activity".)