Italian mathematician (9)
I believe the answer is:
fibonacci
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">italian mathematician</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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" .)