One of five arrivals in small church a supporter there (7)
I believe the answer is:
squinch
'a supporter there' is the definition.
I can't tell whether this defines the answer.
'one of five arrivals in small church' is the wordplay.
'one of five arrivals' becomes 'quin' (short for quintuplet, one of five children).
'in' means one lot of letters goes inside another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'church' becomes 'ch' (common abbreviation).
's'+'ch'='sch'
'quin' going inside 'sch' is 'SQUINCH'.
(Other definitions for squinch that I've seen before include "Screw up" , "arch support" .)