Crossword Genius

Union not to be mentioned (4)

Ross

I believe the answer is:

knot

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

'union' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Maybe there's an association between them I don't understand?

'not to be mentioned' is the wordplay.
'to be mentioned' shows a homophone (sound like).
'not' sounds like 'KNOT'.

Can you help me to learn more?

(Other definitions for knot that I've seen before include "See 2" , "Difficult problem" , "Bird; hard part in wood" , "Unit of speed in water" , "Small sandpiper - fastening device" .)

I've seen this clue in the Evening Standard.
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