File coating (5)
I believe the answer is:
emery
'file coating' 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 powder" , "old comedian" , "Coating on paper used as an abrasive" , "Hard, grey-black mineral used as 11 down" , "Rock used as abrasive" .)