Break back of missal somewhere in church (7)
I believe the answer is:
chancel
'somewhere in church' is the definition.
(part of a church)
'break back of missal' is the wordplay.
'break' becomes 'chance' (as in someone's big break).
'back of' says to take the final letters.
The last letter of 'missal' is 'l'.
'chance'+'l'='CHANCEL'
(Other definitions for chancel that I've seen before include "Part of church near altar" , "Place of altar" , "Where to see choir" , "screened-off area" , "what's behind the screen" .)