Subsequently alter form (5)
I believe the answer is:
later
'subsequently' is the definition.
(I've seen this before)
'alter form' is the wordplay.
'form' indicates anagramming the letters.
'alter' is an anagram of 'LATER'.
(Other definitions for later that I've seen before include "at a subsequent point" , "Afterward, subsequent" , "At a subsequent time" , "Alter (anag.)" , "after the rest?" .)