Fussy, that's certain (10)
I believe the answer is:
particular
This clue is a double definition.
'fussy' is the first definition.
(I've seen this before)
'certain' is the second definition.
(I've seen this before)
'that's' acts as a link.
(Other definitions for particular that I've seen before include "Specific" , "item" , "Fastidious; individual" , "Fog over London" , "Nice" .)