How many letters does the top answer for “move or swing back and forth” have?

The most common answer is brandish (8 letters).

Crossword clue: move or swing back and forth

1 answer found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

brandish 8 letters wn-def

move or swing back and forth

move back and forth or sideways 60% similar → rock, sway

move or cause to move back and forth 60% similar → shake

move back and forth very rapidly 50% similar → flicker, flitter, flutter