There's a fishy redness about it (4)

I believe the answer is:

rudd

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'it' is the definition.
The definition suggests a singular noun which matches the answer.

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(Other definitions for rudd that I've seen before include "Freshwater fish of the carp family" , "Swimmer" , "Freshwater fish (silvery with red fins)" , "European freshwater fish" , "Red-eye fish" .)

I've seen this clue in the Evening Standard.

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