Unruly perms cut in rainbow colours (8)
I believe the answer is:
spectrum
'rainbow colours' is the definition.
'unruly perm's cut' is the wordplay.
'unruly' indicates anagramming the letters.
'perms'+'cut'='permscut'
'permscut' anagrammed gives 'SPECTRUM'.
'in' is the link.
(Other definitions for spectrum that I've seen before include "lot of colours" , "Bands of colour in crumpets" , "Wide range" , "Light split by prism" , "Crumpets (anag)" .)