Relative back in English church (5)
I believe the answer is:
niece
'relative' is the definition.
(niece is a kind of relative)
'back in english church' is the wordplay.
'back' says the letters should be written in reverse.
'english' becomes 'e' (abbreviation).
'church' becomes 'ce' (abbreviation for Church of England).
'in' back-to-front is 'ni'.
'ni'+'e'+'ce'='NIECE'
(Other definitions for niece that I've seen before include "Female relation" , "Family relative" , "Brother's daughter" , "member of different generation" , "Son's cousin" .)