Going to the island church, on a whim (7)
I believe the answer is:
caprice
'whim' is the definition.
(I know that whim can be written as caprice)
'going to the island church' is the wordplay.
'going to' is a charade indicator (letters next to each other).
'the island' becomes 'capri' (Italian island).
'church' becomes 'CE' (abbreviation for Church of England).
'capri' put next to 'ce' is 'CAPRICE'.
'on a' acts as a link.
(Other definitions for caprice that I've seen before include "Whimsy" , "Sudden change of mind" , "A whim, or piece of music in lively fanciful style" , "'Impulse, whim (7)'" , "Sudden mood change" .)