This stuff makes a companion rail twisted (8)
I believe the answer is:
material
'this stuff' is the definition.
(synonyms)
'a companion rail twisted' is the wordplay.
'a companion' becomes 'mate' (both can mean a friend).
'twisted' indicates an anagram.
'rail' with letters rearranged gives 'rial'.
'mate'+'rial'='MATERIAL'
'makes' acts as a link.
(Other definitions for material that I've seen before include "grave" , "Matter; fabric" , "Stuff - relevant" , "Substantial -- textile" , "Cloth, fabric" .)