Mental dullness that would have upset Proust (6)
I believe the answer is:
stupor
'mental dullness that' is the definition.
'stupor' can be an answer for 'mental dullness' (stupor means a dull state of mind). I'm not sure about the rest of the definition.
'upset proust' is the wordplay.
'upset' indicates anagramming the letters.
'proust' is an anagram of 'STUPOR'.
'would have' is the link.
Can you help me to learn more?
(Other definitions for stupor that I've seen before include "Almost unconscious state" , "Daze or torpor" , "Proust found in dazed condition" , "Dazed or unconscious condition, sometimes caused by alcohol" , "Insensible state" .)