Finish revising, but not before noon (6)
I believe the answer is:
ending
'finish' is the definition.
(thesaurus)
'revising but not before noon' is the wordplay.
'revising' becomes 'amending' (amending is a kind of revising).
'but not' is a deletion indicator.
'before noon' becomes 'am'.
'amending' with 'am' taken out is 'ENDING'.
(Other definitions for ending that I've seen before include "Gin den (anag.)" , "Climax" , "Final part" , "terminal" , "Putting a stop to" .)