Fret over some scattered light, eg, diffraction (6)
I believe the answer is:
fidget
'fret' is the definition.
(thesaurus)
'over some scattered light eg diffraction' is the wordplay.
'over' says the letters should be written backwards.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'tegdif' is hidden in the letters of 'scattered light eg diffraction'.
'tegdif' reversed gives 'FIDGET'.
(Other definitions for fidget that I've seen before include "Squirm" , "Move restlessly" , "Wriggle" , "Make small restless movements with hands" , "Make nervous movements of the hands" .)