For that reason, the cover is thick (5)
I believe the answer is:
solid
'thick' is the definition.
(I've seen this before)
'for that reason the cover' is the wordplay.
'for that reason' becomes 'so' (I've seen this in other clues).
'the 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 "Substantial - having three dimensions" , "Strongly made or built" , "Of substance" , "Not hollow, the same throughout" , "continuous" .)