Family member that is immersed in new church (5)
I believe the answer is:
niece
'family member' is the definition.
(I know that family member can be written as niece)
'that is immersed in new church' is the wordplay.
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'immersed in' is an insertion indicator.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'church' becomes 'CE' (abbreviation for Church of England).
'n'+'ce'='nce'
'ie' inserted into 'nce' is 'NIECE'.
(Other definitions for niece that I've seen before include "Family relation" , "Young female relative" , "Female relation" , "One of the family" , "A sibling's daughter" .)