Polychromatic arc (7)
I believe the answer is:
rainbow
'polychromatic arc' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for rainbow that I've seen before include "Arc of colour in the sky" , "Multi-coloured arc in the sky" , "Colourful display" , "Natural arch of colours" , "Spectrum in the sky" .)