In sound English, not the price of vice! (6)
I believe the answer is:
syntax
'in sound english' 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?
'not the price of vice' is the wordplay.
I cannot really see how this works, but
'not' could be 'nt' ('n't' can be a synonym of 'not') and 'nt' is present in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for syntax that I've seen before include "Rules of grammar" , "Rules governing formation of sentences and phrases" , "Grammatical word structure in sentences" , "The rules governing the arranging of words" , "Rules governing the use of words" .)