Spectrum end (3)
I believe the answer is:
red
'spectrum end' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for red that I've seen before include "Colour of a cardinal's hat" , "Primary colour" , "'If you see it, perhaps you're angry (3)'" , "14 -- breed of 4" , "The colour of the former Russian army" .)