Certainly don't be so sceptical (9)

I believe the answer is:

doubtless

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'certainly' is the definition.
(I've seen this before)

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

(Other definitions for doubtless that I've seen before include "Assuredly" , "Probably, certainly" , "of course" , "Definitely not" , "Unquestionably" .)

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