Fresh weight revealed by famous scientist (6)
I believe the answer is:
newton
'famous scientist' is the definition.
(I've seen this before)
'fresh weight' is the wordplay.
'fresh' becomes 'new' (associated in meaning).
'weight' becomes 'ton' (unit of weight).
'new'+'ton'='NEWTON'
'revealed by' is the link.
(Other definitions for newton that I've seen before include "Explainer of gravity" , "Famous scientist" , "He discovered the law of gravity" , "Formulator of the law of gravitation, d. 1727" , "He is said to have discovered gravity" .)