Alter morning final (5)
I believe the answer is:
amend
'alter' is the definition.
(amending is a kind of altering)
'morning final' is the wordplay.
'morning' becomes 'am' (as in 12:00 AM).
'final' becomes 'end' (synonyms).
'am'+'end'='AMEND'
(Other definitions for amend that I've seen before include "Make minor changes to" , "Correct (one's ways)" , "Make minor improvements to (something)" , "Make small changes to" , "Change for the better" .)