How many letters does the top answer for “upright consisting of a vertical side member of a door or window frame” have?

The most common answer is jamb (4 letters).

Crossword clue: upright consisting of a vertical side member of a door or window frame

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

jamb 4 letters wn-def

upright consisting of a vertical side member of a door or window frame