Scientist unfamiliar with nitrogen? (6)
I believe the answer is:
newton
'scientist' is the definition.
(Sir Isaac Newton)
'unfamiliar with nitrogen?' is the wordplay.
'unfamiliar with' becomes 'new to' (I've seen this in another clue).
'nitrogen?' becomes 'N' (N is the chemical symbol for nitrogen).
'newto'+'n'='NEWTON'
(Other definitions for newton that I've seen before include "Formulator of the law of gravitation, d. 1727" , "Great English mathematician who discovered gravity" , "Great English scientist who discovered gravity" , "Scientist - unit of force" , "Sir Isaac --; -- Abbot" .)