Mother’s holding meeting — a source of eruptions (5)
I believe the answer is:
magma
'a source of eruptions' is the definition.
I can't tell whether this definition defines the answer.
'mother's holding meeting' is the wordplay.
'mother' becomes 'ma' (Ma is another word for mother or mum).
'holding' indicates putting letters inside.
'meeting' becomes 'AGM' (annual general meeting in a company).
'ma' going around 'agm' is 'MAGMA'.
Can you help me to learn more?
(Other definitions for magma that I've seen before include "Hot molten rock" , "Hot material" , "hot stuff" , "Molten rock in the earth's crust" , "Lava" .)