Therefore, top must be reliable (5)
I believe the answer is:
solid
'reliable' is the definition.
(thesaurus)
'therefore top' is the wordplay.
'therefore' becomes 'so'.
'top' becomes 'lid' (lid is a kind of top).
'so'+'lid'='SOLID'
'must be' is the link.
(Other definitions for solid that I've seen before include "Firm and stable" , "Concrete or reilable" , "Three-dimensional - compact" , "Not hollow or liquid" , "Neither liquid nor gas" .)