Empty, not solid (6)
I believe the answer is:
hollow
This clue is a double definition.
'empty' is the first definition.
(I know that empty can be written as hollow)
'not solid' is the second definition.
('hollow' is the opposite to 'solid')
(Other definitions for hollow that I've seen before include "Concave - sunken" , "Not solid, having hole in the middle" , "Having an empty space inside" , "Having a cavity inside" , "Empty, not solid" .)