A rebellious vicar’s protest (4)
I believe the answer is:
aver
'protest' is the definition.
(I've seen this before)
'a rebellious vicar's' is the wordplay.
'rebellious' says the letters should be written in reverse.
'vicar' becomes 'rev'.
'rev' back-to-front is 'ver'.
'a'+'ver'='AVER'
(Other definitions for aver that I've seen before include "State firmly - don't rave" , "Declare < old cattle" , "State; say" , "make a statement" , "State firmly, declare" .)