How many letters does the top answer for “addition of extra material or illustration or clarifying detail” have?

The most common answer is amplification (13 letters).

Crossword clue: addition of extra material or illustration or clarifying detail

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

amplification 13 letters wn-def

addition of extra material or illustration or clarifying detail

elaboration 11 letters wn-def

addition of extra material or illustration or clarifying detail