Capital form of punishment (6)
I believe the answer is:
stocks
I believe this is a double definition.
'capital' is the first definition.
(I know this)
'form of punishment' is the second definition.
(Other definitions for stocks that I've seen before include "Village greens used to have these" , "Keeps" , "Plants; form of punishment" , "Reserves and inventories" , "Frame for shipbuilding - form of punishment" .)