Drug problem, so sir can suffer (8)
I believe the answer is:
narcosis
'drug problem' is the definition.
The definition and answer can be both related to thinking as well as being singular nouns.
Maybe you can see an association between them that I don't see?
'so sir can suffer' is the wordplay.
'suffer' indicates anagramming the letters (I've seen 'suffers' mean this).
'so'+'sir'+'can'='sosircan'
'sosircan' anagrammed gives 'NARCOSIS'.
Can you help me to learn more?
(Other definitions for narcosis that I've seen before include "State of stupor" , "Drowsiness produced by a drug" , "result of excess speed?" , "Drug-induced state" , "Drug effect" .)