Coloured arc (7)
I believe the answer is:
rainbow
'coloured arc' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for rainbow that I've seen before include "weather feature" , "All the colours of the spectrum" , "'Somewhere over the ...' (song) (7)" , "heavenly sight" , "'I can sing a - - ' (song)" .)