Small brown songbird (4)
I believe the answer is:
wren
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">small brown songbird</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for wren that I've seen before include "Tiny brown songbird with short erect tail" , "Bird - architect, 1632-1723" , "Tiny bird" , "Tiny brown songbird with a short upright tail" , "Very small brown songbird with erect tail" .)