Point of (ultimately) no return (7)
I believe the answer is:
rubicon
'point of ultimately no return' is the definition.
The answer and definition can be both related to thinking as well as being singular nouns.
Maybe there's an association between them I don't understand?
'point of' is the wordplay.
I cannot really see how this works, but
'point' could be 'n' (North) and 'n' is found within the answer.
'of' could be 'o' and 'o' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for rubicon that I've seen before include "5 significantly crossed!" , "Caesar crossed this river to take irrevocable step" , "cause of conflict when crossed" , "Italy/Gaul boundary" , "No, I curb the fateful river" .)