Killer returned and cut down European in citadel (9)
I believe the answer is:
acropolis
'citadel' is the definition.
(acropolis is a kind of citadel)
'killer returned and cut down european' is the wordplay.
'killer' becomes 'orca' (orca is a term for the killer whale).
'returned' says the letters should be written backwards.
'and' says to put letters next to each other.
'cut down european' becomes 'polis' (I can't explain this - if you can you should give a lot more credence to this answer).
'orca' back-to-front is 'acro'.
'acro'+'polis'='ACROPOLIS'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for acropolis that I've seen before include "Citadel of ancient Greece" , "classical site" , "Upper fortified part of Greek city - Athens's is famous" , "high point" , "place of high security" .)