Very quiet man's initially made a false deduction (7)
I believe the answer is:
sophism
'a false deduction' is the definition.
The definition and answer can be both related to thinking as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'very quiet man's initially made' is the wordplay.
'very' becomes 'so' (I've seen this in other clues).
'quiet' becomes 'p' (music symbol for 'piano').
'man's' becomes 'his'.
'initially made' becomes 'm' (I am not sure about this - if you are sure you should believe this answer much more).
'so'+'p'+'his'+'m'='SOPHISM'
Can you help me to learn more?
(Other definitions for sophism that I've seen before include "faulty reasoning" , "False argument" , "specious argument" .)