Market's worst seller (4)
I believe the answer is:
drug
This might be a double definition.
'market's' is the first definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Maybe you can see a link between them that I can't see?
'worst seller' is the second definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.
Can you help me to learn more?
(Other definitions for drug that I've seen before include "Substance used in medicine, often abused" , "LSD, say" , "Perhaps an upper" , "Tranquillize" , "Medication" .)