Spice in overturned stone cask (6)
I believe the answer is:
nutmeg
'spice' is the definition.
(I know that Nutmeg is a spice)
'overturned stone cask' is the wordplay.
'overturned' shows that the letters should be reversed in order.
'stone' becomes 'gem' (I've seen this in another clue).
'cask' becomes 'tun' (tun is a kind of cask).
'gem'+'tun'='gemtun'
'gemtun' written backwards gives 'NUTMEG'.
'in' acts as a link.
(Other definitions for nutmeg that I've seen before include "Hard seed of tropical tree used as spice" , "One grated as spice" , "Spicy seasoning - hard fruit for little Margaret?" , "Variety of spice" , "spicy stuff" .)