One transformed in time (3)
I believe the answer is:
eon
'time' is the definition.
(I've seen this before)
'one transformed' is the wordplay.
'transformed' indicates an anagram.
'one' anagrammed gives 'EON'.
'in' is the link.
(Other definitions for eon that I've seen before include "Era" , "Long period of time" , "Prolonged period" , "Very long age" , "An immense time" .)