Crossword Genius

Certain people cannot be so described (8)

Ross

I believe the answer is:

doubtful

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'certain' is the definition.
Both the answer and definition are adjectives. Perhaps they are linked in a way I don't understand?

I can't explain the remainder of the clue.

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(Other definitions for doubtful that I've seen before include "Open to question, uncertain" , "description of Thomas" , "Unsettled in mind or opinion" , "Uncertain, suspicious" , "like Thomas" .)

I've seen this clue in The Independent.
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