Sway during themes they're playing, every so often (6)
I believe the answer is:
teeter
'sway' is the definition.
(I've seen this in another clue)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for teeter that I've seen before include "Vacillate" , "Rock precariously on the edge" , "Wobble on the brink" , "Sway, as if about to fall" , "Seesaw, totter" .)