Abrasive substance (5)
I believe the answer is:
emery
'abrasive substance' 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" , "Abrasive powder" , "Hard black-grey mineral used in grinding and polishing" , "Black abrasive" , "old comedian" .)