Unknown listener visiting every month (4,5)
I believe the answer is:
year round
'unknown listener visiting every month' is the definition.
I can't judge whether this defines the answer.
'unknown listener visiting every' is the wordplay.
'unknown' becomes 'y' (algebra).
'listener' becomes 'ear' (body part which hears or listens).
'visiting' is a charade indicator (letters next to each other).
'every' becomes 'round' (I can't explain this - if you can you should give a lot more credence to this answer).
'y'+'ear'+'round'='YEAR-ROUND'
Can you help me to learn more?
(Another definition for year round that I've seen is " every season".)