Bishop’s fitting takes place before mass religious ceremony (7)
I believe the answer is:
baptism
'religious ceremony' is the definition.
(baptism is a kind of religious ceremony)
'bishop's fitting takes place before mass' is the wordplay.
'bishop' becomes 'b' (abbreviation used in chess).
'fitting' becomes 'apt' (I've seen this before).
'takes place before' becomes 'is' (I can't justify this - if you can you should give a lot more credence to this answer).
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'b'+'apt'+'is'+'m'='BAPTISM'
Can you help me to learn more?
(Other definitions for baptism that I've seen before include "Baby's wetting of the head" , "Christian sacrament that symbolises admission to the church" , "Christening ceremony" , "Religious rite of purification with water" , "Christian sacrament requiring pouring of water" .)