Great Swiss mathematician (5)
I believe the answer is:
euler
'great swiss mathematician' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for euler that I've seen before include "Noted blind Swiss mathematician" , "calculating foreigner" , "Swiss mathematician; sounds like lubricator" , "Swiss scientist" , "Leonhard -----, founder of mathematical analysis" .)