Graduates taking head of geography to be hot stuff (5)
I believe the answer is:
magma
'hot stuff' is the definition.
(I've seen this before)
'graduates taking head of geography' is the wordplay.
'graduates' becomes 'MA MA' (more than one holder of a Master of Arts degree).
'taking' indicates putting letters inside.
'head of' says to take the initial letters.
The first letter of 'geography' is 'g'.
'mama' enclosing 'g' is 'MAGMA'.
'to be' acts as a link.
(Other definitions for magma that I've seen before include "crust's content" , "Hot fluid rock" , "Glassy volcanic rock" , "Semi-fluid material within the earth's crust" , "Molten or pasty rock" .)