Smoothing tool (4)
I believe the answer is:
file
'smoothing tool' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for file that I've seen before include "Line on board" , "Dossier; line of people" , "Folder; line" , "Smoothing tool; organised data" , "Tool for sawing through metal" .)