Union that's not sound? (4)
I believe the answer is:
knot
'union' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Maybe you can see an association between them that I can't see?
'not sound?' is the wordplay.
'sound?' shows a homophone (sound like) (I've seen 'sound' mean this).
'not' is a homophone of 'KNOT'.
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for knot that I've seen before include "Nautical mile, loosely" , "complex problem" , "Tie (or what is put in one?)" , "Nautical speed - group of toads" , "Measure of speed at sea" .)