Make a later modification (5)
I believe the answer is:
alter
'make' is the definition.
(thesaurus)
'a later modification' is the wordplay.
'modification' indicates anagramming the letters.
'later' with letters rearranged gives 'ALTER'.
(Other definitions for alter that I've seen before include "and 10 Across: Second self" , "Make changes to" , "Alert to change" , "Vary" , "shift" .)