Mass of rock coming from left? (3)
I believe the answer is:
tor
'mass of rock' is the definition.
'tor' can be an answer for 'rock' (tor is a kind of rock). I am not sure about the 'mass of' bit.
I don't understand how the remainder of the clue works.
(Other definitions for tor that I've seen before include "Hill or rocky peak" , "Fermenting agent" , "It rises" , "Rot on a peak in Devon" , "Rot on a hill in Cornwall" .)