Disagreement heretic enters into versus church (8)
I believe the answer is:
variance
'disagreement' is the definition.
(variance is a kind of disagreement)
'heretic enters into versus church' is the wordplay.
'heretic' becomes 'arian' (Arianism is a form of heresy).
'enters into' indicates putting letters inside.
'versus' becomes 'v'.
'church' becomes 'CE' (Church of England).
'v'+'ce'='vce'
'arian' put inside 'vce' is 'VARIANCE'.
(Other definitions for variance that I've seen before include "Change" , "Dissension" , "Not the main attraction" , "Difference, discrepancy" , "disagreement" .)