Clue I'd set for old mathematician (6)
I believe the answer is:
euclid
'for old mathematician' is the definition.
(ancient Greek mathematician)
'clue i'd set' is the wordplay.
'set' indicates an anagram (instruction to set the letters).
'clue'+'id'='clueid'
'clueid' is an anagram of 'EUCLID'.
(Other definitions for euclid that I've seen before include "Greek philosopher" , "Geometrician of the 3rd century BC" , "mathematician appears" , "Great Greek mathematician, notable for geometry" , "Greek mathematician, 3rd century BC" .)