Clock regulator (8)
I believe the answer is:
pendulum
'clock regulator' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for pendulum that I've seen before include "Oscillating part of clock" , "A swinging weight as in old clock" , "Moves to and fro" , "Swinger in clock" , "Lead swinging" .)