Not hollow (5)
I believe the answer is:
solid
'not hollow' is the definition.
('solid' is the opposite to 'hollow')
This is the entire clue.
(Other definitions for solid that I've seen before include "Firm - three-dimensional" , "Substantial - having three dimensions" , "Three-dimensional shape, not hollow" , "Reliable - substantial - pure" , "Reliable; not hollow" .)