How many letters does the top answer for “a particularly difficult or baffling question or problem” have?

The most common answer is poser (5 letters).

Crossword clue: a particularly difficult or baffling question or problem

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

poser 5 letters wn-def

a particularly difficult or baffling question or problem

stumper 7 letters wn-def

a particularly difficult or baffling question or problem

toughie 7 letters wn-def

a particularly difficult or baffling question or problem