Keeps a device for punishment (6)
I believe the answer is:
stocks
I believe this is a double definition.
'keeps' is the first definition.
(I've seen this before)
'a device for punishment' is the second definition.
(Other definitions for stocks that I've seen before include "Old public punishment device" , "bands that hold up ships" , "Reserves and inventories" , "Securities, investments" , "bloomers" .)