Story of beam reflected by Newton (4)
I believe the answer is:
yarn
'story' is the definition.
(synonyms)
'beam reflected by newton' is the wordplay.
'beam' becomes 'ray' (synonyms).
'reflected' is a reversal indicator (letters reflected back).
'by' says to put letters next to each other.
'newton' becomes 'N' (symbol for the scientific unit of force).
'ray' back-to-front is 'yar'.
'yar'+'n'='YARN'
'of' acts as a link.
(Other definitions for yarn that I've seen before include "Spun thread or wool or tall tale" , "Not fully credible story" , "which could unravel?" , "Story, and thread or wool, say" , "Spun fibre" .)