Peak in Thessaly (4)
I believe the answer is:
ossa
'peak in thessaly' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ossa that I've seen before include "Technical name for bones" , "mountain in Thessaly" , "Legendary mountain" , "To heap Pelion on -- is to compound difficulties" , "So as to turn to bones" .)