Goddess of the rainbow (4)
I believe the answer is:
iris
'goddess of the rainbow' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for iris that I've seen before include "Coloured part of the lamp" , "bit of a looker" , "Bright flower or eye part" , "Plant - part of the eye" , "Film about writer _______ Murdoch" .)