Bad clue I had for mathematician (6)
I believe the answer is:
euclid
'mathematician' is the definition.
(Euclid is an example)
'bad clue i had' is the wordplay.
'bad' is an anagram indicator.
'i had' becomes 'id' (I'd).
'clue' is an anagram of 'eucl'.
'eucl'+'id'='EUCLID'
'for' is the link.
(Other definitions for euclid that I've seen before include "Great Greek mathematician, notable for geometry" , "mathematician appears" , "Greek mathematician, d. mid-3rd century BC" , "Greek father of geometry" , "Old Greek master of geometry" .)