Brother killed rook and flightless birds (5)
I believe the answer is:
remus
'brother killed' is the definition.
(mythical figure killed by his brother Romulus)
'rook and flightless birds' is the wordplay.
'rook' becomes 'R' (chess abbreviation).
'and' says to put letters next to each other.
'flightless birds' becomes 'emus' (emu is a kind of flightless bird).
'r'+'emus'='REMUS'
(Other definitions for remus that I've seen before include "City founder - "uncle" storyteller" , "Brother to fifty two across" , "Twin of Romulus" , "Legendary founder of Rome" , "Famous twin" .)