Swiss mathematician Leonhard (5)
I believe the answer is:
euler
'swiss mathematician leonhard' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for euler that I've seen before include "Eighteen-hole game" , "Leonhard -, C-eighteen Swiss mathematician" , "calculating foreigner" , "See fifty three across" , "Noted blind Swiss mathematician" .)