Back and forth (2,3,3)
I believe the answer is:
to and fro
'back and forth' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for to and fro that I've seen before include "both ways" , "From place to place" , "Directions of oscillation" , "Shuttle's motion" , "Back and forth like pendulum (2.3.3)" .)