A boxed soldier? (6)

I believe the answer is:

sentry

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'a boxed soldier?' is the definition.
The answer and definition can be both people as well as being singular nouns.
Perhaps they are linked in a way I don't understand?

This is the entire clue.

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(Other definitions for sentry that I've seen before include "One who stands guard" , "Soldier on guard" , "Sentinel, look-out" , "Lookout soldier" , "Military guard" .)

I've seen this clue in the Evening Standard.

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