Alter form afterwards (5)
I believe the answer is:
later
'afterwards' is the definition.
(I know that afterwards can be written as later)
'alter form' is the wordplay.
'form' is an anagram indicator (another form of the letters).
'alter' with letters rearranged gives 'LATER'.
(Other definitions for later that I've seen before include "Afterward, subsequent" , "succeeding" , "Following" , "Alter (anag.)" , "In due course" .)