Union Jack, maybe, on vessel (6)

I believe the answer is:

flagon

'vessel' is the definition.
(I know that flagon is a type of vessel)

'union jack maybe on' is the wordplay.
'union jack maybe' becomes 'flag' (the union jack is a flag).
'flag'+'on'='FLAGON'

(Other definitions for flagon that I've seen before include "Drinking vessel" , "Large rounded bottle, may contain cider" , "Large round bottle as for cider" , "Large wine vessel" , "Large drinks container" .)

I've seen this clue in The Guardian.

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