How many letters does the top answer for “author of violin concerto” have?

The most common answer is berg (4 letters).

Crossword clue: author of violin concerto

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

berg 4 letters wd-fact-v3

a large mass of ice floating at sea; usually broken off of a polar glacier

tchaikovsky 11 letters wd-fact-v3

important Russian composer whose works are noted for their expressive melodies (1840-1893)

author of violin concerto in e minor 60% similar → mendelssohn

composer of violin concerto 50% similar → berg, sibelius

wrote violin concerto 50% similar → berg, tchaikovsky