Doubtless, is particular (7)
I believe the answer is:
I believe this clue is a double definition.
'doubtless' is the first definition.
'particular' is the second definition.
(I've seen this before)
'is' is the link.
(Other definitions for certain that I've seen before include "Decided" , "Absolutely sure" , "Particular" , "Unquestionable" , "Convinced" .)