Clue: One of five arrivals in small church a supporter there (7)
Answer: SQUINCH
Explanation: Definition: a supporter there
 QUIN(short for "quintuplet", one of five babies born/arrivals at one birth) **contained in**(in) [ S(abbrev. for "small") + CH(abbrev. for "church") ].
Defn: An arched structure supporting, say, a dome as may be found in a church/"there" in the clue.