Nineties eccentric, brilliant scientist (8)
I believe the answer is:
einstein
'brilliant scientist' is the definition.
(I've seen this before)
'nineties eccentric' is the wordplay.
'eccentric' indicates anagramming the letters.
'nineties' anagrammed gives 'EINSTEIN'.
(Other definitions for einstein that I've seen before include "Set nine up for his theory of relativity" , "US physicist" , "Mathematical genius of nineties" , "Great scientist, founded theory of relativity" , "He contributed the theory of relativity" .)