Cleric fills bucket to prove superior (7)
I believe the answer is:
prevail
'prove superior' is the definition.
(I've seen this before)
'cleric fills bucket' is the wordplay.
'cleric' becomes 'rev' (abbreviation for Reverend).
'fills' is an insertion indicator.
'bucket' becomes 'pail' (synonyms).
'rev' going inside 'pail' is 'PREVAIL'.
'to' acts as a link.
(Other definitions for prevail that I've seen before include "Be victorious" , "Succeed ultimately" , "Have opinion accepted" , "Be widespread; prove superior" , "get the upper hand" .)