Athletic event (9)
'athletic event' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for decathlon that I've seen before include "Sporting event with 10 disciplines" , "Athletics contest" , "Versatile Olympic event" , "Olympic athletics event" , "sporting contest" .)
(Other definitions for triathlon that I've seen before include "Swimming/cycling/running event" , "Contest involving swimming, running and cycling" , "Multi-disciplined Olympic event since 2000" , "Triple athletic event" , "Sporting event" .)