Close market (4)
I believe the answer is:
plug
This clue might be a double definition.
'close' is the first definition.
(I know that plug is a type of close)
'market' is the second definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps you can see a link between them that I can't see?
Can you help me to learn more?
(Other definitions for plug that I've seen before include "It fits into electric socket" , "Blatant promotion" , "Stopper - advertise" , "Bung -- promotion" , "Wad, of tobacco say" .)