Big but not good religious school cut in two (6)
I believe the answer is:
bisect
'cut in two' is the definition.
('bisect' can be a synonym of 'cut in two')
'big but not good religious school' is the wordplay.
'but not' suggests deleting specific letters.
'good' becomes 'g' (abbreviation).
'religious school' becomes 'sect'.
'big' with 'g' taken out is 'bi'.
'bi'+'sect'='BISECT'
(Other definitions for bisect that I've seen before include "equally split" , "Cut in two equal parts" , "Cut equally in two" , "create schism?" , "Cut in half" .)