A cop let off old prostitute (7)

I believe the answer is:

polecat

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'old prostitute' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.

'a cop let off' is the wordplay.
'off' indicates an anagram.
'a'+'cop'+'let'='acoplet'
'acoplet' with letters rearranged gives 'POLECAT'.

Can you help me to learn more?

(Other definitions for polecat that I've seen before include "Malodorous carnivore" , "weaselly type" , "Beast of weasel family" , "weaselly creature" , "Kind of weasel, called skunk in America" .)

I've seen this clue in The Independent.

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