Naysayer checked up (6)
I believe the answer is:
denier
'naysayer' is the definition.
(I've seen this in another clue)
'checked up' is the wordplay.
'checked' becomes 'reined' (to rein something in is to control or check it).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'reined' written backwards gives 'DENIER'.
(Other definitions for denier that I've seen before include "Conspiracist probably" , "One who repudiates" , "measure of fine quality" , "Weight unit by which fineness of yarn is measured" , "Units of weight in silks, nylon and other yarns" .)