Tangle of hair by no means under discussion? (4)

I believe the answer is:

knot

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

'tangle' is the definition.
(I know that tangle can be written as knot)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for knot that I've seen before include "Hard round cross-grained piece in wood" , "To secure < > bird" , "Bird; hard part in wood" , "Bird; tie securely" , "Entangle" .)

I've seen this clue in The Guardian.

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