Spirit to unravel anagram - that's key (8)
I believe the answer is:
armagnac
'spirit' is the definition.
(I've seen this before)
'to unravel anagram that's key' is the wordplay.
'to unravel' indicates an anagram (I've seen 'unravelling' mean this).
'that's key' becomes 'c' (I've seen this in other clues).
'anagram' with letters rearranged gives 'armagna'.
'armagna'+'c'='ARMAGNAC'
(Other definitions for armagnac that I've seen before include "Liquor" , "stronger drink than ale" , "Variety of brandy distilled in district of Gascony SW France" , "Dry brandy from south-west France" , "A gram can for a French brandy" .)