Business meeting with postgraduate about underground rock (5)
I believe the answer is:
magma
'underground rock' is the definition.
(I've seen this in another clue)
'business meeting with postgraduate about' is the wordplay.
'business meeting' becomes 'agm' (abbreviation for annual general meeting).
'with postgraduate' becomes 'ma' (Master of Arts is a postgraduate degree. I am not sure about the 'with' bit.).
'about' is an insertion indicator.
'agm' going into 'ma' is 'MAGMA'.
Can you help me to learn more?
(Other definitions for magma that I've seen before include "That's hot!" , "Molten rock in the earth's crust" , "Hot molten rock" , "crust's content" , "Molten or pasty rock" .)