One alters with age (3)
I believe the answer is:
eon
'age' is the definition.
(I know that eon is a type of age)
'one alters' is the wordplay.
'alters' is an anagram indicator.
'one' with letters rearranged gives 'EON'.
'with' is the link.
(Other definitions for eon that I've seen before include "Major division of geological time" , "plenty of time" , "Immense period" , "Prolonged period" , "Vast age (alt. sp.)" .)