Sun god the origin of yellow beam (3)
I believe the answer is:
ray
'beam' is the definition.
(synonyms)
'sun god the origin of yellow' is the wordplay.
'sun god' becomes 'ra' (Egyptian god).
'the origin of' indicates taking the first letters.
The initial letter of 'yellow' is 'y'.
'ra'+'y'='RAY'
(Other definitions for ray that I've seen before include "Flat fish or sun beam" , "Shaft, beam" , "Pencil of light" , "A bright beam or flat fish" , "Beam or flatfish" .)