Therefore cover is reliable (5)
I believe the answer is:
solid
'reliable' is the definition.
(thesaurus)
'therefore cover' is the wordplay.
'therefore' becomes 'so'.
'cover' becomes 'lid' (lid is a kind of cover).
'so'+'lid'='SOLID'
'is' is the link.
(Other definitions for solid that I've seen before include "Dense" , "Rock-hard" , "Of substance" , "thick" , "Reliable - substantial - pure" .)