Young woman enthralled by hearing organ with unknown agent (8)
I believe the answer is:
Here is my best explanation:
'agent' is the definition.
(I've seen this before)
'young woman enthralled by hearing organ with unknown' is the wordplay.
'young woman' becomes 'miss' ('miss' can be a synonym of 'young woman'**).
'enthralled by' means one lot of letters goes inside another ('enthral' can mean 'capture').
'hearing organ' becomes 'ear' (I've seen this before).
'with' means one lot of letters go next to another.
'unknown' becomes 'y' (algebra).
'miss' inserted within 'ear' is 'emissar'.