Peak in the "Odyssey" (4)
I believe the answer is:
'peak in the odyssey' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ossa that I've seen before include "Legendary mountain" , "So as to become a Greek mountain" , "Hard things" , "So as to turn to bones" , "To pile Pelion upon - is to make even greater" .)