Unknown listener visiting for twelve months (4,5)
I believe the answer is:
year round
'unknown' is the definition.
Both the definition and answer are adjectives. Perhaps there's an association between them I don't understand?
'listener visiting for twelve months' is the wordplay.
'listener visiting' becomes 'round' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'for' means one lot of letters go next to another (I've seen this in other clues).
'twelve months' becomes 'year' (I've seen this in another clue).
'round' put after 'year' is 'YEAR-ROUND'.
Can you help me to learn more?
(Another definition for year round that I've seen is " every season".)