Keeps a device for punishment (6)
I believe the answer is:
Here is my best explanation:
This clue is a double definition.
'keeps' is the first definition.
(I've seen this before)
'device for punishment' is the second definition.
'a' is the link.
(Other definitions for stocks that I've seen before include "heavily-scented flowers", "Keeps", "Amounts of goods kept by traders", "Frame for shipbuilding - form of punishment", "Village greens used to have these", "In which miscreant is locked", "Old public punishment device", "Stores - an old form of punishment", "bloomers" and "stores".)