From side to side (2-3-3)
(Other definitions for to-and-fro that I've seen before include "Back and forth" , "Hither and thither" .)
'from side to side' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for across that I've seen before include "Clues like this" , "Facing" , "written like this?" , "Unlike this clue" , "ax" .)