How many letters does the top answer for “a collection of things sharing a common attribute” have?

The most common answer is category (8 letters).

Crossword clue: a collection of things sharing a common attribute

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

category 8 letters wn-def

a collection of things sharing a common attribute

class 5 letters wn-def

a collection of things sharing a common attribute

sharing of common attributes 60% similar → commonality, commonness