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