Mental confusion (4)
I believe the answer is:
haze
'mental confusion' is the definition.
(haze is a kind of mental confusion)
This is the entire clue.
(Other definitions for haze that I've seen before include "Vapour" , "Lack of definition or precision" , "'Mist, light fog (4)'" , "Daze, fog" , "A thin mist, perhaps from heat" .)