Chopper blade going to and fro (5)
I believe the answer is:
rotor
'chopper blade' is the definition.
(I've seen this before)
'going to and fro' is the wordplay.
I cannot quite see how this works, but
'to' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for rotor that I've seen before include "System of turning helicopter blades" , "Revolving part of turbine" , "Machine part that turns around" , "It goes around" , "Turning motor part" .)