Shaped like a rainbow (5)
I believe the answer is:
arced
'shaped like a rainbow' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for arced that I've seen before include "Moved in a curve" , "Forming a curve" , "Cared about being curved" , "Curvy" , "Moved in curved path" .)