Like ring topped with cool colour (6)
I believe the answer is:
indigo
'colour' is the definition.
(indigo is a kind of colour)
'like ring topped with cool' is the wordplay.
'like' becomes 'dig' (I've seen this in other clues).
'ring' becomes 'o'.
'topped with' means one lot of letters go next to another.
'cool' becomes 'in' (I've seen this in other clues).
'dig'+'o'='digo'
'digo' put after 'in' is 'INDIGO'.
(Other definitions for indigo that I've seen before include "Blue dyestuff" , "type of dye" , "Violet dye" , "Mood - (Duke Ellington)" , "Natural blue dye" .)