English sovereign beheaded calculating Swiss (5)
I believe the answer is:
euler
'calculating swiss' is the definition.
(Swiss mathematician Leonhard Euler)
'english sovereign beheaded' is the wordplay.
'english' becomes 'e' (abbreviation).
'sovereign' becomes 'ruler' (I've seen this before).
'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 "Leonhard -, C-eighteen Swiss mathematician" , "Leonhard -, Eighteenth century Swiss mathematician" , "18th-century Swiss mathematician and physicist" , "Noted blind Swiss mathematician" , "See fifty three across" .)