Saint occupying extremely religious office (6)
I believe the answer is:
vestry
'religious office' is the definition.
(office in a church)
'saint occupying extremely' is the wordplay.
'saint' becomes 'st' (abbreviation).
'occupying' indicates putting letters inside (inserted letters occupy or enter the word).
'extremely' becomes 'very' (very can mean extremely or considerably).
'st' inserted inside 'very' is 'VESTRY'.
(Other definitions for vestry that I've seen before include "Church robing-room" , "Part of church used as an office" , "Small room attached to church" , "meeting of parishioners" , "Room in a church where sacred vessels and vestments are kept" .)