Shows how thick stockings are, but he refuses to accept the evidence (6)
I believe the answer is:
denier
'shows how thick stockings are but he refuses to accept the evidence' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'he refuses to accept the evidence' is the wordplay.
I cannot quite see how this works, but
'he' could be 'den' and 'den' is present in the answer.
The remaining letters 'ier' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for denier that I've seen before include "Peter?" , "Units of weight in silks, nylon and other yarns" , "how fine stockings are?" , "A measure of material" , "Unit of measurement for silk" .)