How many letters does the top answer for “showing sustained enthusiastic action with unflagging vitality” have?

The most common answer is indefatigable (13 letters).

Crossword clue: showing sustained enthusiastic action with unflagging vitality

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

indefatigable 13 letters wn-def

showing sustained enthusiastic action with unflagging vitality

unwearying 10 letters wn-def

showing sustained enthusiastic action with unflagging vitality