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", "Particular", "Unquestionable", "Indisputable", "Sure, definite".)