Very unstable conical explosive (8)
I believe the answer is:
volcanic
'explosive' is the definition.
(thesaurus)
'very unstable conical' is the wordplay.
'very' becomes 'v' (abbreviation).
'unstable' is an anagram indicator.
'conical' with letters rearranged gives 'olcanic'.
'v'+'olcanic'='VOLCANIC'
(Other definitions for volcanic that I've seen before include "Given to violent eruption like natural force" , "Liable to erupt" , "Of Etna or Vesuvius?" , "Liable to violent outbursts" , "Describing lava?" .)