Brother killed rook and flightless birds (5)
I believe the answer is:
remus
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">brother killed</span>' is the definition.<br/ ><span class="explanation-format__explanation">(mythical figure killed by his brother Romulus)</span><br/ ><br/ >'<span class="explanation-format__original">rook and flightless birds</span>' is the wordplay.<br/ >'<span class="explanation-format__original">rook</span>' becomes '<span class="explanation-format__intermediate">R</span>' <span class="explanation-format__explanation">(chess abbreviation)</span>.<br/ >'<span class="explanation-format__original">and</span>' says to put letters next to each other.<br/ >'<span class="explanation-format__original">flightless birds</span>' becomes '<span class="explanation-format__intermediate">emus</span>' <span class="explanation-format__explanation">(emu is a kind of flightless bird)</span>.<br/ >'<span class="explanation-format__intermediate">r</span>'+'<span class="explanation-format__intermediate">emus</span>'='<span class="explanation-format__answer">REMUS</span>'<br/ ></span>
(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" .)