Mountain in Greek mythology sandwiched between Olympus and Pelion by Otus and Ephialtes in an attempt to reach heaven (4)
I believe the answer is:
ossa
(Other definitions for ossa that I've seen before include "To pile Pelion upon - is to make even greater" , "So as to become a Greek mountain" , "To heap Pelion on -- is to compound difficulties" , "So as to turn to bones" , "Technical name for bones" .)