English ruler beheaded mathematician (5)
I believe the answer is:
euler
'mathematician' is the definition.
(I've seen this before)
'english ruler beheaded' is the wordplay.
'english' becomes 'e' (abbreviation).
'beheaded' means to remove the first letter (the head of the word is removed).
'ruler' with its first letter taken away is 'uler'.
'e'+'uler'='EULER'
(Other definitions for euler that I've seen before include "See fifty three across" , "Leonhard -, Eighteenth century Swiss mathematician" , "Swiss mathematician; sounds like lubricator" , "18th-century Swiss mathematician and physicist" , "Swiss scientist" .)