Knock over stone cask to get spice (6)
I believe the answer is:
nutmeg
'spice' is the definition.
(nutmeg is a kind of spice)
'knock over stone cask' is the wordplay.
'knock over' shows that the letters should be reversed in order.
'stone' becomes 'gem' (I've seen this before).
'cask' becomes 'tun' (tun is a kind of cask).
'gem'+'tun'='gemtun'
'gemtun' back-to-front is 'NUTMEG'.
'to get' is the link.
(Other definitions for nutmeg that I've seen before include "Aromatic seed for grating" , "Variety of spice" , "Asian tree whose aromatic seed is used as spice when grated or ground" , "Hard fruit and girl for a spice" , "Spicy seasoning - hard fruit for little Margaret?" .)