After a fire, this can cause darkness (4)

I believe the answer is:

soot

I'm a little stuck... Click here to teach me more about this clue!

'after a fire this can cause darkness' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.

This is all the clue.

Can you help me to learn more?

(Other definitions for soot that I've seen before include "Stuff from chimneys" , "Fine carbon deposit" , "Something stuck down the chimney" , "Black chimney powder" , "Residue from burning" .)

I've seen this clue in the Evening Standard.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.