Certain people cannot be so described (8)
I believe the answer is:
doubtful
'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.
Can you help me to learn more?
(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" .)