Underground lava (5)
I believe the answer is:
magma
'underground lava' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for magma that I've seen before include "Hot fluid rock" , "Glassy volcanic rock" , "Hot material" , "Molten rock in the earth's crust" , "Semi-fluid material within the earth's crust" .)