Oriental king beheaded mathematician (5)
I believe the answer is:
euler
'mathematician' is the definition.
(I've seen this before)
'oriental king beheaded' is the wordplay.
I cannot really see how this works, but
'oriental' could be 'e' and 'e' is present in the answer.
'king' could be 'er' (abbreviation for Edward Rex, various English kings) and 'er' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for euler that I've seen before include "Leonhard -----, founder of mathematical analysis" , "Leonhard --, C18 Swiss mathematician" , "calculating foreigner" , "Leonhard -, Eighteenth century Swiss mathematician" , "See fifty three across" .)