Dawn reveals this insult about United Nations (8)
I believe the answer is:
sunlight
'dawn' is the definition.
(thesaurus)
'this insult about united nations' is the wordplay.
'this insult' becomes 'slight' (I've seen this before).
'about' indicates putting letters inside.
'united nations' becomes 'UN' (abbreviation).
'slight' going around 'un' is 'SUNLIGHT'.
'reveals' acts as a link.
(Other definitions for sunlight that I've seen before include "Prerequisite for photosynthesis" , "port?" , "Solar rays" , "Solar illumination" , "Form of illumination" .)