In this way the cover becomes substantial (5)
I believe the answer is:
solid
'substantial' is the definition.
(I know that substantial can be written as solid)
'in this way the cover' is the wordplay.
'in this way' becomes 'so'.
'the cover' becomes 'lid' (lid is a kind of cover).
'so'+'lid'='SOLID'
'becomes' is the link.
(Other definitions for solid that I've seen before include "continuous" , "Sound; substantial" , "Weighty" , "Reliable - strong" , "Not hollow, the same throughout" .)