Not a sound union? (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.
Perhaps there's a link between them I don't understand?
'not a sound' is the wordplay.
'a sound' indicates a 'sounds like' (homophone) clue.
'not' is a homophone of 'KNOT'.
Can you help me to learn more?
(Other definitions for knot that I've seen before include "bond, perhaps?" , "Ring < sandpiper" , "Fastening such as reef or granny" , "Tangled mass" , "Unit of length used in navigation" .)