Razor sharpener (5)
'razor sharpener' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for strop that I've seen before include "Paddy" , "Fit of bad temper" , "Strip of leather on which razor is sharpened" , "Bad temper - sharpen (a razor)" , "temper tantrum" .)