Ray and Barb outside the United Nations (8)
I believe the answer is:
sunlight
'ray' is the definition.
(I've seen this before)
'barb outside the united nations' is the wordplay.
'barb' becomes 'slight' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'outside' is an insertion indicator (some letters go outside others).
'the united nations' becomes 'UN' (abbreviation).
'slight' going around 'un' is 'SUNLIGHT'.
'and' acts as a link.
Can you help me to learn more?
(Other definitions for sunlight that I've seen before include "What's illuminating, naturally" , "Beams" , "Prerequisite for photosynthesis" , "Hustling under heavenly radiance" , "Form of illumination" .)