Flag seen with gin cocktail (5,6)

I believe the answer is:

white ensign

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'flag' is the definition.
Both the answer and definition are singular nouns.
Maybe there's a link between them I don't understand?

'seen with gin cocktail' is the wordplay.
'cocktail' indicates an anagram.
'seen'+'with'+'gin'='seenwithgin'
'seenwithgin' anagrammed gives 'WHITE ENSIGN'.

Can you help me to learn more?

(Other definitions for white ensign that I've seen before include "Royal Navy flag" , "standard at sea" .)

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