How many letters does the top answer for “slow to learn or understand; lacking intellectual acuity” have?

The most common answer is dense (5 letters).

Crossword clue: slow to learn or understand; lacking intellectual acuity

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

dense 5 letters wn-def

slow to learn or understand; lacking intellectual acuity

obtuse 6 letters wn-def

slow to learn or understand; lacking intellectual acuity