Eventually alter form (5)
I believe the answer is:
later
'eventually' is the definition.
(I've seen this in another clue)
'alter form' is the wordplay.
'form' indicates an anagram.
'alter' anagrammed gives 'LATER'.
(Other definitions for later that I've seen before include "Not just yet" , "goodbye" , "Afterward, subsequent" , "Eventually" , "In due course" .)