Coloured arc in the sky (7)
I believe the answer is:
rainbow
'coloured arc in the sky' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for rainbow that I've seen before include "Arc caused by refraction of the sun's rays (7" , "colours in the sky" , "illusory hope?" , "Arc of colours" , "Multicoloured atmospheric phenomenon" .)