How many letters does the top answer for “see 2d sicker, sickerly, etc.” have?

The most common answer is siker (5 letters).

Crossword clue: see 2d sicker, sickerly, etc.

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

siker 5 letters web-def

Sure; certain; trusty. [Obs. or Prov. Eng. & Scot.] Burns. When he is siker of his good name. Chaucer.

sikerly 7 letters web-def

Surely; securely. [Obs.] But sikerly, withouten any fable. Chaucer.

sikerness 9 letters web-def

The quality or state of being sicker, or certain. [Obs.] Chaucer. Spenser.