Flat-topped cool? (5-10)

I believe the answer is:

level-headedness

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

'cool?' is the definition.
Both the definition and answer are singular nouns.
Maybe there's an association between them I don't understand?

'flat-topped' is the wordplay.
I cannot really understand how this works, but
'flat' could be 'level' (synonyms) and 'level' is present in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

I've seen this clue in The Telegraph.

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.