Wren or robin (4)
I believe the answer is:
bird
'wren or robin' is the definition.
(I know that jenny wren is a type of passeriform bird)
This is the entire clue.
(Other definitions for bird that I've seen before include "Fowl" , "Feathered creature" , "Flying creature" , "Nest builder?" , "Member of class Aves" .)