How many letters does the top answer for “running in an opposite direction.” have?

The most common answer is countercurrent (14 letters).

Crossword clue: running in an opposite direction.

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

countercurrent 14 letters web-def

actions counter to the main group activity

in the opposite direction 67% similar → counter

turning in the opposite direction 50% similar → turnabout, turnaround