Molten rock expelled by a volcano (4)
I believe the answer is:
lava
'molten rock expelled by a volcano' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for lava that I've seen before include "Molten volcanic rock" , "Stuff heatedly hurled out" , "What volcano ejects" , "Outflow from volcano" , "Hot flowing rock" .)