Graduates take German to be hot stuff (5)
I believe the answer is:
magma
'be hot stuff' is the definition.
(molten rock)
'graduates take german' is the wordplay.
'graduates' becomes 'MA MA' (more than one holder of a Master of Arts degree).
'take' is an insertion indicator.
'german' becomes 'g' (abbreviation).
'mama' placed around 'g' is 'MAGMA'.
'to' is the link.
(Other definitions for magma that I've seen before include "Hot molten rock" , "Molten or pasty rock" , "Hot fluid rock" , "hot stuff" , "crust's content" .)