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' is an anagram indicator.
'one' with letters rearranged gives 'EON'.
'in' acts as a 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" .)