Crossword Genius

Vessel - showing its nationality? (6)

Ross

I believe the answer is:

flagon

I'm a little stuck... Click here to teach me more about this clue!

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

'showing its nationality?' is the wordplay.
I cannot quite see how this works, but
'showing' could be 'on' (as in a film showing or on at a cinema) and 'on' is found in the answer.
The remaining letters 'flag' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for flagon that I've seen before include "Large wine-bottle, jug" , "Large vessel with a handle and spout" , "Wine jug" , "Large container for serving drink" , "Large drinks container" .)

I've seen this clue in The Globe and Mail.
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