Modern, heavyweight physicist (6)
I believe the answer is:
newton
'physicist' is the definition.
(Sir Isaac Newton)
'modern heavyweight' is the wordplay.
'modern' becomes 'new' (synonyms).
'heavyweight' becomes 'ton' (a ton weighs a lot).
'new'+'ton'='NEWTON'
(Other definitions for newton that I've seen before include "Sir Isaac --; -- Abbot" , "Famous mathematician" , "Famous scientist" , "Polymath who stated the laws of gravity and motion" , "inclined to apply discovery?" .)