75% of prison is empty (6)
I believe the answer is:
hollow
'is empty' is the definition.
(synonyms)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for hollow that I've seen before include "Pyrrhic" , "Not solid, having hole in the middle" , "Empty - hole" , "Having an empty space inside" , "Having a cavity inside" .)