Mercury will go down with it - so may people (4)
I believe the answer is:
cold
'mercury will' is the definition.
The answer and definition can be both related to thinking as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'go down with it so may people' is the wordplay.
I cannot quite understand how this works, but
'down' could be 'd' and 'd' is found in the answer.
The remaining letters 'col' is a valid word which might be clued in a way I don't see.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for cold that I've seen before include "Unsympathetic" , "Lacking affection" , "Common ailment" , "Mild viral infection, very common" , "Glacial" .)