Subsequently alter form (5)
I believe the answer is:
later
'subsequently' is the definition.
(subsequently means later on)
'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 "at a subsequent point" , "Afterward, subsequent" , "At a subsequent time" , "Alter (anag.)" , "after the rest?" .)