It's right in church to perform part of service in Latin (5)
I believe the answer is:
credo
'part of service in latin' is the definition.
(statement of Christian belief recited in Latin)
'it's right in church to perform' is the wordplay.
'it's right' becomes 'R' (common abbreviation).
'in' means one lot of letters goes inside another.
'church' becomes 'CE' (abbreviation for Church of England).
'to perform' becomes 'do' ('do' can be a synonym of 'perform').
'r' placed into 'ce' is 'cre'.
'cre'+'do'='CREDO'
(Other definitions for credo that I've seen before include "conviction" , "One's system of beliefs" , "music in church" , "A statement of one's beliefs" , "set of ideas" .)