How many letters does the top answer for “to sing in concert. shak.” have?

The most common answer is quire (5 letters).

Crossword clue: to sing in concert. shak.

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

quire 5 letters web-def

a quantity of paper; 24 or 25 sheets