Move or swing from side to side regularly (9)
I believe the answer is:
oscillate
'move or swing from side to side regularly' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for oscillate that I've seen before include "shake" , "seesaw" , "sway" , "Move or swing from side to side regularly" , "Swing like a pendulum" .)