The art of dealing with the upper set, maybe (9)
I believe the answer is:
dentistry
'the art' is the definition.
The definition and answer can be both related to thinking as well as being singular nouns.
Perhaps you can see a link between them that I don't see?
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for dentistry that I've seen before include "It's trendy to engage in care in teeth" , "Work which is looking down in the mouth?" , "It's trendy in profession of teeth-care" , "Practice relating to teeth" , "Professional care of teeth" .)