Theorist in nineties, surprisingly (8)
I believe the answer is:
einstein
'theorist' is the definition.
(Albert Einstein, theory of relativity)
'nineties surprisingly' is the wordplay.
'surprisingly' is an anagram indicator (a surprising ordering of the letters).
'nineties' with letters rearranged gives 'EINSTEIN'.
'in' acts as a link.
(Other definitions for einstein that I've seen before include "Great scientist, founded theory of relativity" , "Albert -, famous scientist" , "German-born physicist" , "Father of Relativity" , "Iconic theoretical physicist, d. 1955" .)