How many letters does the top answer for “an occurrence of something” have?

The most common answer is case (4 letters).

Crossword clue: an occurrence of something

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

case 4 letters wn-def

an occurrence of something

instance 8 letters wn-def

an occurrence of something

the relative frequency of occurrence of something 50% similar → incidence