Be a rotter? (5)

I believe the answer is:

decay

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'be a rotter?' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.

This is the entire clue.

Can you help me to learn more?

(Other definitions for decay that I've seen before include "Decompose or fall into ruin" , "Briefly fashionable" , "Rot, perish" , "Putrefy" , "Rot, moulder" .)

I've seen this clue in the Evening Standard.

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