Crossword Genius

Messenger decapitated by good shot (6)

Ross

I believe the answer is:

gunner

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'good shot' is the definition.
(thesaurus)

I cannot understand how the rest of the clue works.

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(Other definitions for gunner that I've seen before include "Private in the Royal Artillery" , "Artilleryman" , "Artillery soldier" .)

I've seen this clue in The Independent.
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