Vicar overlooks fool in church making crack (8)
I believe the answer is:
crevasse
'crack' is the definition.
(crevasse is a kind of crack)
'vicar overlooks fool in church' is the wordplay.
'vicar' becomes 'rev'.
'overlooks' means one lot of letters go next to another (down clues only).
'fool' becomes 'ass' (ass is a kind of fool**).
'in' indicates putting letters inside.
'church' becomes 'ce' (abbreviation for Church of England).
'rev'+'ass'='revass'
'revass' inserted within 'ce' is 'CREVASSE'.
'making' is the link.
(Other definitions for crevasse that I've seen before include "gap" , "cold break" , "big split" , "Crack in a mountain slope or a glacier" , "Sever sac (anag)" .)