How many letters does the top answer for “form an arch or curve” have?

The most common answer is arc (3 letters).

Crossword clue: form an arch or curve

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

arc 3 letters wn-def

electrical conduction through a gas in an applied electric field

form an arch over 67% similar → overarch

the exterior curve of an arch 50% similar → extrados

the interior curve of an arch 50% similar → intrados