In teens, I fancied I was a genius (8)
I believe the answer is:
einstein
'i was a genius' is the definition.
(Albert Einstein)
'in teens i fancied' is the wordplay.
'fancied' indicates anagramming the letters (a fancy or special version of the letters).
'in'+'teens'+'i'='inteensi'
'inteensi' with letters rearranged gives 'EINSTEIN'.
(Other definitions for einstein that I've seen before include "German expert on relativity" , "Albert, who founded the theory of relativity" , "Brilliant physicist" , "Famous scientist of the nineties" , "Albert -, famous scientist" .)