How many letters does the top answer for “an agent that gives or restores life or vigor” have?

The most common answer is invigorator (11 letters).

Crossword clue: an agent that gives or restores life or vigor

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

invigorator 11 letters wn-def

an agent that gives or restores life or vigor

quickener 9 letters wn-def

an agent that gives or restores life or vigor

give new life or vigor to 50% similar → revitalise