How many letters does the top answer for “consisting of only one morpheme” have?

The most common answer is monomorphemic (13 letters).

Crossword clue: consisting of only one morpheme

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

monomorphemic 13 letters wn-def

consisting of only one morpheme

consisting of one sphere only. 60% similar → monospherical

consisting of only one type 60% similar → monotypic

a composition consisting of one verse only. 50% similar → monostich