Someone who says no - how thick (6)
I believe the answer is:
denier
I believe this clue is a double definition.
'someone who says no' is the first definition.
(someone who denies or says no)
'how thick' is the second definition.
(denier is a measure of thickness in material)
(Other definitions for denier that I've seen before include "One refuses to acknowledge" , "A measure of material" , "Measurement of fineness in silk, nylon etc" , "Measure of the fineness of silk or nylon" , "Peter?" .)