Stroke of a whip (4)
I believe the answer is:
lash
'stroke of a whip' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for lash that I've seen before include "Whip; attack verbally" , "Bind tightly with a rope or cord" , "Birch" , "over the eye" , "Hit with a whip" .)