Irony etc, overturning outside a harbour (7)

I believe the answer is:

seaport

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'a harbour' is the definition.
('seaport' can be a synonym of 'harbour')

I don't understand the rest of the clue.

(Other definitions for seaport that I've seen before include "Town with deep-water harbour" , "Embarkation and disembarkation point for ocean-going ships" , "Lisbon, for example" , "Town with a large harbour" , "Esparto (anag)" .)

I've seen this clue in The Telegraph.

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