Swing to and fro (4)
'swing to and fro' 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 "Move back and forth" , "Ways to waver in opinion or posture" , "Control" , "Vacillate" , "Totter; influence" .)
(Other definitions for flap that I've seen before include "Fluster" , "Moving surface on wing" , "Minor panic; tab" , "State of panic" , "Panic; fold" .)