On which to advertise toothpaste? (4)
I believe the answer is:
tube
'toothpaste?' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps you can see an association between them that I don't see?
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for tube that I've seen before include "Underground transport" , "Cylindrical hollow conduit" , "Valve" , "appearing tinny?" , "channel" .)