Old scientist in novel put on weight (6)
I believe the answer is:
newton
'old scientist' is the definition.
(I know that Newton is a physicist)
'novel put on weight' is the wordplay.
'novel' becomes 'new' (novel means new or fresh).
'put on' means one lot of letters go next to another.
'weight' becomes 'ton' (unit of weight).
'new'+'ton'='NEWTON'
'in' acts as a link.
(Other definitions for newton that I've seen before include "Sir Isaac -, seventeen to eighteenth century English scientist" , "SI unit of force" , "Thandie - - , actress" , "Famous mathematician" , "Famous scientist" .)