Molten rock mother found crossing top of gorge (5)
I believe the answer is:
magma
'molten rock' is the definition.
(molten rock from a volcano)
'mother found crossing top of gorge' is the wordplay.
'mother' becomes 'mama' (mama informally means mother).
'found crossing' is an insertion indicator (some letters cross round others).
'top of' indicates taking the first letters.
The initial letter of 'gorge' is 'g'.
'mama' enclosing 'g' is 'MAGMA'.
(Other definitions for magma that I've seen before include "Hot material" , "hot stuff" , "Lava" , "hot filling in crust" , "Hot fluid rock" .)