Old soldier with his back up? (6)

I believe the answer is:

archer

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'old soldier' is the definition.

I don't understand how the remainder of the clue works.

(Other definitions for archer that I've seen before include "A bow-and-arrow man" , "Toxophilist" , "Her car is targeted by one with a bow and arrow" , "A 19 lord - old soldier" , "Irish parliament" .)

I've seen this clue in the Evening Standard.

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