Head of maths first seen with a man? Wrong (8)
I believe the answer is:
mistaken
'wrong' is the definition.
(similar in meaning)
'head of maths first seen with a man?' is the wordplay.
'head of' indicates taking the first letters.
'first' becomes 'ist' (ist resembles kst).
'seen with' says to put letters next to each other.
'man?' becomes 'ken' (short for Kenneth).
The initial letter of 'maths' is 'm'.
'm'+'ist'+'a'+'ken'='MISTAKEN'
(Other definitions for mistaken that I've seen before include "Wrongly understood" , "'Wrong, misguided (8)'" , "Wrong in opinion, judgment etc." , "Wrong (in judgement)" , "In error in mankiest way" .)