Molten rock in the earth's crust (5)
I believe the answer is:
magma
'molten rock in the earth's crust' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for magma that I've seen before include "Molten rock in the earth's crust" , "Lava" , "hot stuff" , "Glassy volcanic rock" , "Hot fluid rock" .)