Ian follows a number of birds (5)
I believe the answer is:
avian
'of birds' is the definition.
(I've seen this before)
'ian follows a number' is the wordplay.
'follows' says to put letters next to each other.
'a number' becomes 'av' (I've seen this in other clues).
'ian' put after 'av' is 'AVIAN'.
(Other definitions for avian that I've seen before include "Like rails, for example" , "Resembling a bird" , "To do with birds - like a dangerous flu" , "Relating to birds" , "Regarding birds" .)