Crossword Genius

After British sent packing, gracious in imprisoning singular officer (6)

Ross

I believe the answer is:

ensign

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'officer' is the definition.
(I've seen this before)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for ensign that I've seen before include "Military or naval flag" , "Soldier who carries the Colours" , "National flag" , "US naval rank" , "Ship's flag and junior officer" .)

I've seen this clue in The Independent.
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