Article is starting to transform a believer (6)
I believe the answer is:
theist
'a believer' is the definition.
(theist is a kind of believer)
'article is starting to transform' is the wordplay.
'article' becomes 'the' (the 'definite article' part of speech).
'starting to' indicates taking the first letters.
The first letter of 'transform' is 't'.
'the'+'is'+'t'='THEIST'
(Other definitions for theist that I've seen before include "One believes in the existence of a god or gods" , "Believer in a god or gods" , "Believer in god/gods" , "Person who believes in the existence of a god" , "Religious type" .)