Six into articles about birds (5)
I believe the answer is:
avian
'about birds' is the definition.
(avian means relating to birds)
'six into articles' is the wordplay.
'six' becomes 'vi' (roman numeral).
'into' means one lot of letters goes inside another.
'articles' becomes 'aan' (two examples of the part-of-speech: a+an).
'vi' going into 'aan' is 'AVIAN'.
(Other definitions for avian that I've seen before include "Like rails, for example" , "Pertaining to or characteristic of birds" , "Regarding birds" , "To do with birds - like a dangerous flu" , "Resembling a bird" .)