Aromatic kernel in stone cask overturned (6)
I believe the answer is:
nutmeg
'aromatic kernel' is the definition.
(I've seen this before)
'stone cask overturned' is the wordplay.
'stone' becomes 'gem' (gemstone).
'cask' becomes 'tun' (tun is a kind of cask).
'overturned' is a reversal indicator.
'gem'+'tun'='gemtun'
'gemtun' back-to-front is 'NUTMEG'.
'in' acts as a link.
(Other definitions for nutmeg that I've seen before include "Asian tree whose aromatic seed is used as spice when grated or ground" , "(In soccer) kick the ball between an opponent's legs" , "Aromatic seed - met gnu (anag)" , "Aromatic spicy kernel" , "Aromatic seasoning" .)