One changes in time (3)
I believe the answer is:
eon
'time' is the definition.
(I've seen this before)
'one changes' is the wordplay.
'changes' is an anagram indicator.
'one' anagrammed gives 'EON'.
'in' is the link.
(Other definitions for eon that I've seen before include "Very long period (US and technical spelling)" , "plenty of time" , "Geological period" , "Vast age; eternity" , "Long period of time" .)