Colour of the rainbow (6)
I believe the answer is:
indigo
'colour of the rainbow' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for indigo that I've seen before include "type of dye" , "One of the colours of the rainbow" , "Deep-blue dye or colour" , "Mood - (Duke Ellington)" , "Spectral colour" .)