What does the cryptic crossword clue "A topless marathon? (3)" mean?

The definition is "A". The wordplay uses charade, deletion. RACE (synonym of "marathon?") [beheading: "topless"]; definition: "A" The answer is ACE.

A topless marathon? (3)

Independent Cryptic #11731 | 8 Down | 2024-05-16 flower