Greek mountain’s huge appeal (4)
I believe the answer is:
'greek mountain's' is the definition.
(I've seen this before)
'huge appeal' is the wordplay.
'huge' becomes 'os' (abbreviation for outsize).
'appeal' becomes 'sa' (old-fashioned abbreviation for sex appeal).
(Other definitions for ossa that I've seen before include "So as to become a Greek mountain" , "To heap Pelion on -- is to compound difficulties" , "Legendary mountain" , "Hard things" , "Technical name for bones" .)