Relating to birds (5)
I believe the answer is:
avian
'relating to birds' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for avian that I've seen before include "Like rails, for example" , "Resembling a bird" , "Relating to birds" , "Regarding birds" , "Pertaining to or characteristic of birds" .)