Not conversant with name for a mathematician and physicist (6)
I believe the answer is:
newton
'a mathematician and physicist' is the definition.
(Isaac Newton)
'not conversant with name' is the wordplay.
'not conversant with' becomes 'newto' (as in a phrase like 'he's new to technology').
'name' becomes 'n' (common abbreviation e.g. on forms).
'newto'+'n'='NEWTON'
'for' is the link.
(Other definitions for newton that I've seen before include "Scientist - unit of force" , "English mathematician and physicist, d. 1727" , "Sir Isaac . . . . . ., great English mathematician" , "SI unit of force" , "Supposedly, an apple fell on his head" .)