How many letters does the top answer for “a word or phrase that particular people use in particular situations” have?

The most common answer is locution (8 letters).

Crossword clue: a word or phrase that particular people use in particular situations

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

locution 8 letters wn-def

a word or phrase that particular people use in particular situations

saying 6 letters wn-def

a word or phrase that particular people use in particular situations