Soldier who keeps himself to himself? (7)

I believe the answer is:

private

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'soldier' is the definition.
(private is a type of soldier)

I don't understand the rest of the clue.

(Other definitions for private that I've seen before include "Lowest rank in the army" , "Secret, not public" , "retiring" , "Lowest rank of enlisted person" , "Unofficial; military rank" .)

I've seen this clue in The Irish Independent.

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