Sense master's upset by new rating (12)
I believe the answer is:
reassessment
'new rating' is the definition.
The answer and definition can be both related to thinking as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
'sense master's upset' is the wordplay.
'upset' indicates an anagram.
'sense'+'masters'='sensemasters'
'sensemasters' is an anagram of 'REASSESSMENT'.
'by' acts as a link.
Can you help me to learn more?
(Other definitions for reassessment that I've seen before include "extra test?" , "another property valuation" , "Further appraisal" , "Process of re-evaluation" , "Further thoughts" .)