Crossword Genius

Vulgar laugh that's emitted by brass section from below (3-3)

Ross

I believe the answer is:

haw-haw

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

'vulgar laugh' is the definition.
'haw-haw' can be an answer for 'laugh' (haw-haw is a kind of laugh). I am unsure of the 'vulgar' bit.

I don't understand the remainder of the clue.

Can you help me to learn more?

(Another definition for haw-haw that I've seen is " Sound of a loud laugh".)

I've seen this clue in The Guardian.
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