Banishment from unisex pool arranged after dropping a zip (9)

I believe the answer is:

expulsion

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'banishment' is the definition.
(I know that expulsion is a type of banishment)

I cannot understand how the rest of the clue works.

(Other definitions for expulsion that I've seen before include "Removal, as from school" , "Action of forcing out" .)

I've seen this clue in The Telegraph.

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