How many letters does the top answer for “talk incessantly and tiresomely” have?

The most common answer is jaw (3 letters).

Crossword clue: talk incessantly and tiresomely

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

jaw 3 letters wn-def

the part of the skull of a vertebrate that frames the mouth and holds the teeth

yack 4 letters wn-def

noisy talk

someone who talks incessantly 50% similar → motormouth