Naughty Nineties physicist (8)
I believe the answer is:
einstein
'physicist' is the definition.
(I know that Einstein is a physicist)
'naughty nineties' is the wordplay.
'naughty' indicates an anagram.
'nineties' anagrammed gives 'EINSTEIN'.
(Other definitions for einstein that I've seen before include "Disney's 14 [GREAT DANE]" , "Famous scientist of the nineties" , "Albert -, famous scientist" , "E=mc2 physicist" , "Set nine up for his theory of relativity" .)