Ex-pupil’s girl endlessly digesting current form of rock (8)
I believe the answer is:
obsidian
'form of rock' is the definition.
'obsidian' can be an answer for 'rock' (obsidian is a kind of rock). I'm not certain of the 'form of' bit.
'ex-pupil's girl endlessly digesting current' is the wordplay.
'ex-pupil's' becomes 'OB's' (abbreviation for 'Old Boy').
'girl endlessly' becomes 'dian' (I can't explain this - if you can you should give a lot more credence to this answer).
'digesting' is an insertion indicator (inserted letters are eaten or ingested).
'current' becomes 'I' (symbol for electric current).
'obs'+'dian'='obsdian'
'obsdian' placed around 'i' is 'OBSIDIAN'.
Can you help me to learn more?
(Other definitions for obsidian that I've seen before include "Volcanic glass-like rock" , "Dark volcanic rock" , "Dark volcanic glass" , "Dark glassy rock formed from lava" , "A glassy volcanic rock" .)