Wear down by friction (6)
I believe the answer is:
abrade
'wear down by friction' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for abrade that I've seen before include "Day breaks? > Start as before" , "Scratch" , "Shave" , "Scrape away, erode" , "Wear away by rubbing (6)" .)