Popular credit card, it's turning up in town, say, mostly (7)
I believe the answer is:
plastic
'popular credit card' is the definition.
The definition and answer can be both substances as well as being singular nouns.
Perhaps you can see a link 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 plastic that I've seen before include "Credit cards etc" , "Credit cards are known as ....... money" , "Credit card - pliable" , "Credit card; mouldable" , "Fashionable - substitute for wood or metal" .)