Fine grit (4)
I believe the answer is:
sand
'fine grit' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for sand that I've seen before include "Result of rock erosion" , "Egg timer grains" , "George - - (French authoress)" , "Sort of paper, piper, castle or martin" , "George -, Fr. author" .)