Italian mathematician (9)
I believe the answer is:
fibonacci
'italian mathematician' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for fibonacci that I've seen before include "Mathematician giving name to a series" , "Maths whiz" , "Thirteenth century Italian mathematician associated with series one, one, two, three, five, eight etc" , "Italian mathematician who gave name to sequence" .)