No danger of fog, it's understood (3,5)

I believe the answer is:

all clear

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

'understood' is the definition.
(I've seen this before)

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

(Other definitions for all clear that I've seen before include "I understand everything" , "Understood" , "Siren sound after air raid has ceased" , "no doubts" , "Message confirming safety" .)

I've seen this clue in The Guardian.

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.