Flag officer once? (6)

I believe the answer is:

ensign

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This could be a double definition.

'flag' is the first definition.
(I know that ensign is a type of flag)

'officer once?' is the second definition.
'ensign' can be an answer for 'officer' (I've seen this before). I'm not certain of the 'once?' bit.

Can you help me to learn more?

(Other definitions for ensign that I've seen before include "Look upon with contempt" , "Ship's flag to show nationality" , "US naval rank" , "Junior US naval officer" , "Flying" .)

I've seen this clue in The Times.

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