It goes round and round and to and fro (5)
I believe the answer is:
rotor
'it goes round' is the definition.
(I've seen this before)
'round and to and fro' is the wordplay.
I cannot really understand how this works, but
'to' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
'and' is the link.
Can you help me to learn more?
(Other definitions for rotor that I've seen before include "Arm moving" , "... and another" , "Revolving aerofoil" , "Device turning" , "Rotating part of machine" .)