Coloured arc (7)
I believe the answer is:
rainbow
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">coloured arc</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(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)" .)