Molten rock mother observed around foot of berg (5)
I believe the answer is:
magma
'molten rock' is the definition.
(I've seen this before)
'mother observed around foot of berg' is the wordplay.
'mother' becomes 'mama' (mama is a kind of mother).
'observed around' means one lot of letters goes inside another.
'foot of' says to take the final letters.
The last letter of 'berg' is 'g'.
'mama' going around 'g' is 'MAGMA'.
(Other definitions for magma that I've seen before include "Hot fluid rock" , "Semi-fluid material within the earth's crust" , "Hot material" , "That's hot!" , "Molten rock in the earth's crust" .)