Old punishments may still provide interest (6)
I believe the answer is:
stocks
'old' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for stocks that I've seen before include "Capital holdings often accompanied by shares" , "Keeps" , "Plants; form of punishment" , "Supplies of goods in store" , "Securities, investments" .)