Swiss mathematician (5)
I believe the answer is:
euler
'swiss mathematician' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for euler that I've seen before include "Leonhard --, C18 Swiss mathematician" , "Leonhard -, C-eighteen Swiss mathematician" , "Leonhard -----, founder of mathematical analysis" , "calculating foreigner" , "Swiss scientist" .)