Union not to be mentioned (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 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" .)