Article is written by head of monastery: 'A form of belief' (6)
I believe the answer is:
theism
'belief' is the definition.
(theism is a kind of belief)
'article is written by head of monastery a form' is the wordplay.
'article' becomes 'the' (the 'definite article' part of speech**).
'written by' is a charade indicator (letters next to each other).
'head of' indicates taking the first letters.
'a form' indicates anagramming the letters.
The initial letter of 'monastery' is 'm'.
'is' after 'm' is 'mis'.
'mis' anagrammed gives 'ism'.
'the'+'ism'='THEISM'
'of' acts as a link.
(Other definitions for theism that I've seen before include "of faith" , "The doctrine or belief in the existence of a God or gods" , "Belief in a creator" , "Belief in a deity" , "Belief in existence of a God revealed to man" .)