Nail-filing board (5)
I believe the answer is:
emery
'nail-filing board' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for emery that I've seen before include "Abrasive material" , "Black abrasive" , "Hard, grey-black mineral used as 11 down" , "Hard grey-black mineral used as an abrasive" , "Abrasive form of corundum" .)