He's beside himself when beaten! (3)

I believe the answer is:

tom

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'he's' is the definition.
(I've seen this in another clue)

I can't explain the rest of the clue.

(Other definitions for tom that I've seen before include "Queen's mate" , "Male beast" , "Boy - cat" , "Boy running off with pig" , "He is associated with Dick and Harry" .)

I've seen this clue in the Evening Standard.

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