Source of light in darkness, unbelievable (3)
I believe the answer is:
sun
'source of light' is the definition.
(thesaurus)
'in darkness unbelievable' is the wordplay.
'in' says the answer is hidden in the clue.
'SUN' is hidden in the letters of 'darkness unbelievable'.
(Other definitions for sun that I've seen before include "Major star" , "Dawn riser" , "Star round which we orbit" , "Shiny" , "Centre of a solar system" .)