Old woman takes control of shareholders' meeting that might become a bit rocky (5)

I believe the answer is:
magma
'old woman takes control of shareholders' meeting that might become a bit rocky' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'old woman takes control of shareholders' meeting' is the wordplay.
'old woman' becomes 'ma' (both can mean someone's mother).
'takes control of' is an insertion indicator.
'shareholders' meeting' becomes 'agm' (abbreviation for annual general meeting).
'ma' enclosing 'agm' is 'MAGMA'.
Can you help me to learn more?
(Other definitions for magma that I've seen before include "Semi-fluid material within the earth's crust" , "hot filling in crust" , "Hot molten rock" , "Glassy volcanic rock" , "That's hot!" .)
