Young woman enthralled by hearing organ with unknown agent (8)
I believe the answer is:
Here is my best explanation:
'agent' is the definition.
'young woman enthralled by hearing organ with unknown' is the subsidiary indication.
'young woman' becomes 'miss' ('miss' can be a synonym of 'young woman'**).
'enthralled by' indicates putting letters inside (\'enthral\' can mean \'capture\').
'hearing organ' becomes 'ear' (I've seen this before).
'with' is a charade indicator (letters next to each other).
'unknown' becomes 'y' (algebra).
'miss' inserted inside 'ear' is 'emissar'.
(Other definitions for emissary that I've seen before include "Government messenger", "Diplomatic messenger", "Legate", "Someone sent on a mission", "Representative sent on a mission", "Spy", "Diplomat sent on special task", "Representative on special mission", "Person sent on a diplomatic mission" and "Delegate".)