All the colours of the rainbow (8)
I believe the answer is:
spectrum
'all the colours of the rainbow' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for spectrum that I've seen before include "Broad range of related items" , "Range of colours, opinions etc" , "range of tones" , "Array, range" , "Bands of colour in crumpets" .)