Naturally it can make things a sight worse (3)
I believe the answer is:
fog
'sight worse' is the definition.
The definition and answer can be both related to thinking as well as being singular nouns.
Maybe they are linked in a way I don't understand?
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for fog that I've seen before include "Bank restricting visibility" , "Obscure" , "Miasma" , "Bafflement" , "Impenetrable mist" .)