Analyse grammar written by English Zoroastrian (6)
I believe the answer is:
parsee
'zoroastrian' is the definition.
(I have seen 'A Zoroastrian ' mean 'parsee' so perhaps 'zoroastrian' could also mean 'parsee')
'analyse grammar written by english' is the wordplay.
'analyse grammar' becomes 'parse'.
'written by' is a charade indicator (letters next to each other).
'english' becomes 'e' (abbreviation).
'parse'+'e'='PARSEE'
(Other definitions for parsee that I've seen before include "Adherent of a Zoroastrian sect - a spree (anag)" , "Old immigrant" , "Zoroastrian descendant" , "See rap turned into a Zoroastrian Indian" , "A spree, strangely, following Prophet Zoroaster" .)