Reason why Australian is in Church? (5)
I believe the answer is:
cause
'reason why' is the definition.
(I've seen this in another clue)
'australian is in church?' is the wordplay.
'australian' becomes 'aus' (common abbreviation).
'is in' is an insertion indicator.
'church?' becomes 'CE' (abbreviation for Church of England).
'aus' put inside 'ce' is 'CAUSE'.
(Other definitions for cause that I've seen before include "why some people fight" , "Producer of an effect" , "Reason; belief" , "A movement or charity- usually good" , "Give rise to" .)