Part of a rainbow (6)
I believe the answer is:
indigo
'part of a rainbow' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for indigo that I've seen before include "Mood - (Duke Ellington)" , "Settlement; New Mexico native" , "Blue/violet dye" , "Ellington's Mood" , "Deep dark blue" .)