A good sort but certainly not brilliant (5,7)

I believe the answer is:

rough diamond

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'a good sort' is the definition.
(thesaurus)

I don't understand the rest of the clue.

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

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

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