English monarch beheaded calculating Swiss (5)
I believe the answer is:
euler
'calculating swiss' is the definition.
(Swiss mathematician Leonhard Euler)
'english monarch beheaded' is the wordplay.
'english' becomes 'E' (abbreviation).
'monarch' becomes 'ruler' (I've seen this before).
'beheaded' means to remove the first letter (the head of the word is removed).
'ruler' with its initial letter removed is 'uler'.
'e'+'uler'='EULER'
(Other definitions for euler that I've seen before include "Leonhard --, C18 Swiss mathematician" , "Eighteen-hole game" , "calculating foreigner" , "See fifty three across" , "Swiss scientist" .)