Revise fourth letter following word of agreement (5)
I believe the answer is:
amend
'revise' is the definition.
(I know that amend is a more specific form of revise)
'fourth letter following word of agreement' is the wordplay.
'fourth letter' becomes 'd' (in the alphabet).
'following' says to put letters next to each other.
'word of agreement' becomes 'amen' (I've seen this before).
'd' after 'amen' is 'AMEND'.
(Other definitions for amend that I've seen before include "Put right" , "Make minor improvements to (something)" , "Change, alter" , "Make corrections to (eg, document)" , "Change for the better" .)