Sharp side of a blade (4)
I believe the answer is:
edge
'sharp side of a blade' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for edge that I've seen before include "Rim, lip" , "Move slowly" , "'Border, boundary, rim (4)'" , "Sharpened side of blade" , "Rim - move gradually" .)