Vessel sporting an ensign (6)

I believe the answer is:

flagon

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'vessel' is the definition.
(flagon is a kind of vessel)

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

(Other definitions for flagon that I've seen before include "Large vessel for beverages" , "Large wine vessel" , "Snow White companion" , "Large rounded bottle, may contain cider" , "Large round bottle as for cider" .)

I've seen this clue in The Irish Independent.

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