Cryptic clue I had for mathematician (6)
I believe the answer is:
euclid
'for mathematician' is the definition.
(Euclid is an example)
'cryptic clue i had' is the wordplay.
'cryptic' indicates an anagram (a cryptic or obscure order of the letters).
'i had' becomes 'i'd' (I'd).
'clue' anagrammed gives 'eucl'.
'eucl'+'id'='EUCLID'
(Other definitions for euclid that I've seen before include "Book of geometry" , "Alexandrian mathematician" , "mathematician appears" , "6 mathematician" , "Ancient Greek mathematician" .)