A brown one is no rainbow! (5)

I believe the answer is:

trout

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'rainbow' is the definition.
(I've seen this before)

I don't understand the remainder of the clue.

(Other definitions for trout that I've seen before include "Tasty fish, brown or rainbow" , "one of a noted quintet" , "Fish - old harridan" , "Unattractive old woman" , "one often tickled?" .)

I've seen this clue in the Evening Standard.

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