Having checked back, he says it's not so (6)
I believe the answer is:
denier
'having checked back he says it's not so' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'he says it's not so' is the wordplay.
I cannot quite see how this works, but
'he' could be 'den' and 'den' is found in the answer.
'it' could be 'e' ('e' can mean 'electronic' which is similar to 'IT') and 'e' is present in the answer.
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 "Unit of weight of nylon, etc" , "Measurement of fineness in silk, nylon etc" , "Measure of the fineness of silk or nylon" , "measure of fine quality" , "how fine stockings are?" .)