It's not cricket trousers worn here (5,3,4)

I believe the answer is:

below the belt

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'it's not cricket' is the definition.
(thesaurus)

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

(Other definitions for below the belt that I've seen before include "Where unfair blow lands" , "Illegal" , "Underhand, unfair" , "Hang scabbard here" , "fly is here" .)

I've seen this clue in The Telegraph.

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