Something like a rainbow? (5)

I believe the answer is:

trout

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

I can't explain the remainder of the clue.

(Other definitions for trout that I've seen before include "Schubert work; fish" , "Swimmer" , "one often tickled?" , "unpleasant old woman" , "bad-tempered woman" .)

I've seen this clue in the Evening Standard.

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