A Greek heretic dealing with land (8)
I believe the answer is:
agrarian
'dealing with land' is the definition.
(agrarian means relating to farming)
'a greek heretic' is the wordplay.
'greek' becomes 'gr'.
'heretic' becomes 'arian' (Arianism is a form of heresy).
'a'+'gr'+'arian'='AGRARIAN'
(Other definitions for agrarian that I've seen before include "Relating to cultivated land" , "Of agricultural land or its cultivation" , "Relating to rural matters" , "of the Earth"" , "Concerned with land management" .)