Type of trout is naturally colourful (7)
I believe the answer is:
rainbow
'type of trout' is the definition.
(I've seen this before)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for rainbow that I've seen before include "Natural arch of colours" , "Colourful arc in the sky" , "Heavenly phenomenon" , "Spectral display" , "Colourful display" .)