Of birds (5)
I believe the answer is:
avian
'of birds' is the definition.
(avian means relating to birds)
This is all the clue.
(Other definitions for avian that I've seen before include "Relating to birds" , "Lizard with adhesive foot-pads" , "Like a booby" , "Pertaining to or characteristic of birds" , "Regarding birds" .)