He lent fresh weight to ideas about physics (6)
I believe the answer is:
newton
'he' is the definition.
'lent fresh weight to ideas about physics' is the wordplay.
I cannot really understand how this works, but
'fresh' could be 'new' (similar in meaning) and 'new' is found in the answer.
'weight' could be 'ton' (unit of weight) and 'ton' is present in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for newton that I've seen before include "He is said to have discovered gravity" , "Englishman known for his work on gravity and motion" , "Gravity theorist" , "Unit of force - English polymath, d. 1727" , "Thandie - - , actress" .)