Rock rhythmically (4)
I believe the answer is:
sway
'rock rhythmically' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sway that I've seen before include "Vacillate" , "Rock to and fro" , "Rule; move to and fro" , "Lean from side to side - influence" , "Totter; influence" .)