A topless marathon? (3)
I believe the answer is:
ace
'a topless marathon?' is the definition.
The answer and definition can be both acts as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'topless marathon?' is the wordplay.
'topless' means to remove the first letter.
'marathon?' becomes 'race' (marathon is a kind of race).
'race' with its first letter removed is 'ACE'.
Can you help me to learn more?
(Other definitions for ace that I've seen before include "Outstanding" , "golfer's achievement" , "Crack" , "Great" , "Single unit on playing cards" .)