Article by naturalist, primarily about birds (5)
I believe the answer is:
avian
'about birds' is the definition.
(avian means relating to birds)
'article by naturalist primarily' is the wordplay.
'article' becomes 'a' (the indefinite article in English).
'by' becomes 'via' (I've seen this in another clue).
'primarily' suggests taking the first letters.
The initial letter of 'naturalist' is 'n'.
'a'+'via'+'n'='AVIAN'
(Other definitions for avian that I've seen before include "Resembling a bird" , "To do with birds - like a dangerous flu" , "Lizard with adhesive foot-pads" , "Like a booby" , "Relating to birds" .)