Mathematician Turing (4)
I believe the answer is:
alan
'mathematician turing' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for alan that I've seen before include "Sir -- Sugar (The Apprentice)" , "Messrs Ladd and Dukes" , "- Bennett, English actor, dramatist, writer" , "One of Robin Hood's men was - a Dale" , "Boy's name" .)