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A worthy person, as yet lacking polish (5,7)

Ross

I believe the answer is:

rough diamond

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'a worthy' is the definition.
(thesaurus)

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

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(Other definitions for rough diamond that I've seen before include "Good guy with uncouth manner" , "Unsophisticated chap - uncut stone" , "Geezer" , "Good but uneducated person" , "a trump, but low" .)

I've seen this clue in The Guardian and The Globe and Mail.
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