Colour of the spectrum (6)
(Other definitions for orange that I've seen before include "tree" , "An ogre (anag.)" , "shade" , "Dutch royal house" , "Mandarin?" .)
'colour of the spectrum' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for indigo that I've seen before include "A deep blue dye or colour" , "Blue/violet dye" , "Ellington's Mood" , "Spectral colour" , "Violet dye" .)
(Other definitions for violet that I've seen before include "Seldom" , "It nods on Oberon's bank" , "Colour at opposite end of spectrum from red" , "A flower for a girl" , "Rainbow colour; flower" .)