Purple robes do for upper-class swinger (8)
I believe the answer is:
pendulum
'swinger' is the definition.
(something which swings)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for pendulum that I've seen before include "Lead swinging" , "Oscillating mechanism" , "It swings to and fro as in old clock" , "Clock regulator" , "Oscillating part of clock" .)