Il Duce turned out to be a mathematician (6)
I believe the answer is:
euclid
'a mathematician' is the definition.
(Euclid is an example)
'il duce turned out' is the wordplay.
'turned out' indicates anagramming the letters.
'il'+'duce'='ilduce'
'ilduce' anagrammed gives 'EUCLID'.
'to be' acts as a link.
(Other definitions for euclid that I've seen before include "Geometrician of the 3rd century BC" , "Alexandrian mathematician" , "6 mathematician" , "Greek philosopher" , "Early Greek geometrician" .)