This idea unacceptable, let me show you and say why (6)
I believe the answer is:
heresy
'this idea unacceptable let me' is the definition.
The definition and answer can be both related to thinking as well as being singular nouns.
Maybe there's an association between them I don't understand?
'show you and say why' is the wordplay.
I cannot quite see how this works, but
'show' could be 'er' (US medical drama) and 'er' is found in the answer.
'why' could be 'y' and 'y' is located in the answer.
The remaining letters 'hes' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for heresy that I've seen before include "Yes her (anag.)" , "Heterodoxy" , "Opinion contrary to accepted religious belief" , "Opinion opposed to orthodox belief" , "religious belief not generally accepted" .)