How many letters does the top answer for “a mark of a foot or shoe on a surface” have?

The most common answer is footmark (8 letters).

Crossword clue: a mark of a foot or shoe on a surface

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

footmark 8 letters wn-def

a mark of a foot or shoe on a surface

footprint 9 letters wn-def

a mark of a foot or shoe on a surface

the under surface of the foot. 50% similar → pelma