Opportunity to go over large part of church (7)
I believe the answer is:
chancel
'part of church' is the definition.
(area of a church)
'opportunity to go over large' is the wordplay.
'opportunity' becomes 'chance' (synonyms).
'to go over' means one lot of letters go next to another (in a down clue, some letters go on top of others).
'large' becomes 'L' (eg in clothes sizes).
'chance'+'l'='CHANCEL'
(Other definitions for chancel that I've seen before include "screened-off area" , "Part of church near altar" , "Where to see choir" , "church, partly" , "Clergy's area of the church" .)