Change one’s mind regarding church fast (6)
I believe the answer is:
relent
'change one's mind' is the definition.
(to relent is to give in or change one's mind)
'regarding church fast' is the wordplay.
'regarding' becomes 're' ('re' means regarding or about).
'church fast' becomes 'lent' (fast in the Christian church).
're'+'lent'='RELENT'
(Other definitions for relent that I've seen before include "Capitulate" , "Ease off" , "Soften - yield" , "Become less severe, soften" , "Moderate" .)