A saint in church group (5)
I believe the answer is:
caste
'group' is the definition.
(an ethnic or social group)
'a saint in church' is the wordplay.
'saint' becomes 'st'.
'in' means one lot of letters goes inside another.
'church' becomes 'CE' (abbreviation for Church of England).
'a'+'st'='ast'
'ast' inserted into 'ce' is 'CASTE'.
(Other definitions for caste that I've seen before include "Hereditary class (in India)" , "Level in society" , "place in society" , "Group within society" , "Religious or social class" .)