How many letters does the top answer for “a blow with the open hand; a slap.” have?

The most common answer is spank (5 letters).

Crossword clue: a blow with the open hand; a slap.

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

spank 5 letters web-def

a slap with the flat of the hand

a hitter who slaps with an open hand 60% similar → slapper

a blow with the hand. 50% similar → handystroke