Explosively unstable (8)
I believe the answer is:
volcanic
'explosively unstable' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for volcanic that I've seen before include "Of lava-producing fissures" , "Relating to fissures which produce lava" , "Like lava, perhaps" , "given to sudden outbursts" , "Given to violent eruption like natural force" .)