Mathematician from Spain developed rule (5)
I believe the answer is:
euler
'mathematician' is the definition.
(I've seen this before)
'spain developed rule' is the wordplay.
'spain' becomes 'e' (abbreviation for Espana).
'developed' indicates anagramming the letters.
'rule' is an anagram of 'uler'.
'e'+'uler'='EULER'
'from' is the link.
(Other definitions for euler that I've seen before include "Leonhard -----, founder of mathematical analysis" , "Eighteen-hole game" , "Leonhard -, Eighteenth century Swiss mathematician" , "Swiss mathematician; sounds like lubricator" , "calculating foreigner" .)