Relative occasionally invited to join church (5)
I believe the answer is:
niece
'relative' is the definition.
(type of relative)
'occasionally invited to join church' is the wordplay.
'occasionally' means one should take alternating letters (letters taken at regular intervals or occasions).
'to join' is a charade indicator (letters next to each other).
'church' becomes 'CE' (Church of England).
The alternating letters of 'invited' are 'nie'.
'nie'+'ce'='NIECE'
(Other definitions for niece that I've seen before include "girl further down the line" , "Brother's daughter" , "Criminal" , "Kinswoman" , "[PRIEST'S] daughter?" .)