Period of time Einstein transformed (8)
I believe the answer is:
nineties
'period of time' is the definition.
(nineties is a kind of period of time)
'einstein transformed' is the wordplay.
'transformed' is an anagram indicator.
'einstein' is an anagram of 'NINETIES'.
(Other definitions for nineties that I've seen before include "Time" , "make batsmen nervous" , "many old people are in" , "possibly last period of life" , "19th century's "naughty" decade" .)