Therefore the top is not hollow (5)
I believe the answer is:
solid
'not hollow' is the definition.
('solid' is the opposite to 'hollow')
'therefore the top' is the wordplay.
'therefore' becomes 'so' (I've seen this in another clue).
'the top' becomes 'lid' (lid is a kind of top).
'so'+'lid'='SOLID'
'is' acts as a link.
(Other definitions for solid that I've seen before include "Substantial - having three dimensions" , "Reliable - substantial - pure" , "Firm, strong" , "Three-dimensional - compact" , "thick" .)