Girl interrupts listener, unknown agent (8)
I believe the answer is:
emissary
'agent' is the definition.
(I've seen this before)
'girl interrupts listener unknown' is the wordplay.
'girl' becomes 'miss' (synonyms).
'interrupts' means one lot of letters goes inside another.
'listener' becomes 'ear' (body part which hears or listens).
'unknown' becomes 'y' (algebra).
'miss' put inside 'ear' is 'emissar'.
'emissar'+'y'='EMISSARY'
(Other definitions for emissary that I've seen before include "Diplomatic messenger" , "Diplomatic representative" , "one dispatched officially" , "Legate" , "Person sent on a diplomatic mission" .)