Crossword Genius

When fish turns up like this, it's rotten (3)

Ross

I believe the answer is:

bad

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'this it's rotten' is the definition.
(I've seen this before)

I don't understand how the remainder of the clue works.

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(Other definitions for bad that I've seen before include "Mouldy, off" , "Of poor quality" , "Ill" , "Substandard" , "(Of food) decayed" .)

I've seen this clue in the Evening Standard.
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