Betray Resistance straightaway when abandoned by Church (3,2)
I believe the answer is:
rat on
'betray' is the definition.
(synonyms)
'resistance straightaway when abandoned by church' is the wordplay.
'resistance' becomes 'r' (electronics abbreviation).
'straightaway' becomes 'at once' (both can mean immediately).
'when abandoned by' is a deletion indicator.
'church' becomes 'CE' (abbreviation for Church of England).
'atonce' with 'ce' removed is 'aton'.
'r'+'aton'='RAT ON'
(Other definitions for rat on that I've seen before include "grass up" , "shop" , "Betray" , "desert" , "Give away" .)