Piece of rough material that could be smoother (5,5)
I believe the answer is:
emery paper
I believe this is a double definition.
'piece of rough material' is the first definition.
(I've seen this before)
'could be smoother' is the second definition.
(I've seen this before)
'that' is the link.
(Other definitions for emery paper that I've seen before include "Sheet covered with mineral powder, used for polishing" , "Piece of rough material" , "I polish" , "could be smoother" , "Abrasive material" .)