Can it lead to a fathead scoring at the wrong end? (3)

I believe the answer is:

fog

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'end?' is the definition.
The answer and definition can be both related to thinking as well as being singular nouns.
Perhaps they are linked in a way I don't understand?

I don't understand how the rest of the clue works.

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(Other definitions for fog that I've seen before include "Bank restricting visibility" , "Obscure" , "Confusion" , "Pea-souper" , "Heavy mist" .)

I've seen this clue in the Evening Standard.

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