Collapse after losing one marathon? (3)
I believe the answer is:
run
'marathon?' is the definition.
(marathon is a kind of run)
'collapse after losing one' is the wordplay.
'collapse' becomes 'ruin' (I've seen this before).
'after losing' suggests deleting specific letters.
'one' becomes 'i' (Roman numeral).
'ruin' with 'i' taken out is 'RUN'.
(Other definitions for run that I've seen before include "Enter a race" , "Compete in race or election" , "Cricket score; go" , "journey" , "A single" .)