In colour it is one of the greats (5)
I believe the answer is:
titan
'one of the greats' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
'in colour it' is the wordplay.
'in' means one lot of letters goes inside another.
'colour' becomes 'tan' (tan is a kind of colour).
'tan' placed around 'it' is 'TITAN'.
'is' acts as a link.
Can you help me to learn more?
(Other definitions for titan that I've seen before include "One of the mythological Greek giants who ruled the world" , "A son of Uranus and Gaea" , "Person of heroic stature" , "Giant god of Gr. myth" , "Goddess" .)