Novel not returned to scientist (6)
I believe the answer is:
newton
'to scientist' is the definition.
(Sir Isaac Newton)
'novel not returned' is the wordplay.
'novel' becomes 'new' (similar in meaning).
'returned' says the letters should be written in reverse.
'not' backwards is 'ton'.
'new'+'ton'='NEWTON'
(Other definitions for newton that I've seen before include "Englishman known for his work on gravity and motion" , "Sir Isaac, discoverer of gravity" , "Great English mathematician who discovered gravity" , "Isaac . . . . . ., great English physicist who discovered gravity" , "Sir Isaac -, seventeen to eighteenth century English scientist" .)