Vermin? What a nuisance! (4)

I believe the answer is:

rats

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'vermin?' is the definition.
(I've seen this in another clue)

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

(Other definitions for rats that I've seen before include "(Hunts) rodents - expression of irritation" , "Snoopy's imprecation - vermin" , "Hamelin's pests" , "deserters" , "Curse" .)

I've seen this clue in the Evening Standard.

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