This person's system of principles rejects English body of stories (9)
I believe the answer is:
mythology
'body of stories' is the definition.
'this person's system of principles rejects english' is the wordplay.
'this person's' becomes 'my' (belonging to the person who set the crossword).
'system of principles' becomes 'theology'.
'rejects' is a deletion indicator.
'english' becomes 'e' (abbreviation).
'theology' with 'e' removed is 'thology'.
'my'+'thology'='MYTHOLOGY'
(Other definitions for mythology that I've seen before include "Study of ancient tales of gods and heroes" , "Body of old fables" , "Legend" , "Legends and sagas, in general" .)