Mental dullness (6)
I believe the answer is:
stupor
'mental dullness' is the definition.
(stupor means a dull state of mind)
This is the entire clue.
(Other definitions for stupor that I've seen before include "State of shock" , "Inertia results!" , "State of insensibility or near-unconsciousness" , "Semi-comatose state" , "Semi-conscious state" .)