How many letters does the top answer for “the subject matter of a conversation or discussion” have?

The most common answer is theme (5 letters).

Crossword clue: the subject matter of a conversation or discussion

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

theme 5 letters wn-def

the subject matter of a conversation or discussion

topic 5 letters wn-def

the subject matter of a conversation or discussion