Inverted box, essential to trap rodent (7)

I believe the answer is:

muskrat

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'rodent' is the definition.
(I know that muskrat is a type of rodent)

I can't explain the rest of the clue.

(Other definitions for muskrat that I've seen before include "UK, smart? (anag)" , "Vole-like creature" , "Rodent valued for its fur" , "Large aquatic rodent" , "Beaver-like rodent with glossy brown fur" .)

I've seen this clue in The Telegraph.

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