Small brown songbird (4)
I believe the answer is:
wren
'small brown songbird' is the definition.
(I've seen this in another clue)
This is all the clue.
(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" .)