Swiss mathematician turned up on toboggan? No good (5)
I believe the answer is:
euler
'swiss mathematician' is the definition.
(I've seen this before)
'turned up on toboggan? no good' is the wordplay.
'turned up' says the letters should be written in reverse.
'on' becomes 're' ('on' can be mean 'with reference to'**).
'toboggan?' becomes 'luge' ('luge' can be a synonym of 'toboggan').
'no' indicates named letters should be taken away.
'good' becomes 'g' (abbreviation).
'luge' with 'g' taken away is 'lue'.
're'+'lue'='relue'
'relue' in reverse letter order is 'EULER'.
(Other definitions for euler that I've seen before include "Swiss mathematician; sounds like lubricator" , "18th-century Swiss mathematician and physicist" , "Eighteen-hole game" , "Leonhard --, C18 Swiss mathematician" , "Leonhard -----, founder of mathematical analysis" .)