One changes in time (3)
I believe the answer is:
eon
'time' is the definition.
(eon is a kind of time)
'one changes' is the wordplay.
'changes' indicates anagramming the letters.
'one' is an anagram of '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" .)