One to grasp decisive point (8)
I believe the answer is:
clincher
'point' is the definition.
The definition and answer can be both related to thinking as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'one to grasp decisive' is the wordplay.
'one to grasp' becomes 'clinch' (clinch is a kind of grasp).
'decisive' becomes 'er' (I can't justify this - if you can you should believe this answer much more).
'clinch'+'er'='CLINCHER'
Can you help me to learn more?
(Other definitions for clincher that I've seen before include "Something settling a matter conclusively" , "Something decisive" , "Decisive argument" , "Decisive factor" , "Argument-settling fact" .)