Man of Letters dismisses a monster in Split? (4)

I believe the answer is:

left

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'split?' is the definition.
(I've seen this before)

I cannot understand how the rest of the clue works.

(Other definitions for left that I've seen before include "Communist?" , "quit" , "political group" , "Punch, perhaps" , "political grouping" .)

I've seen this clue in The Irish Times.

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