Can the old play in this event? (9)
I believe the answer is:
decathlon
'this event?' is the definition.
(decathlon is a kind of event)
'can the old play' is the wordplay.
'play' indicates an anagram.
'can'+'the'+'old'='cantheold'
'cantheold' is an anagram of 'DECATHLON'.
'in' acts as a link.
(Other definitions for decathlon that I've seen before include "Series of events" , "Sporting event with 10 disciplines" , "Athletic contest with ten different events" , "Versatile Olympic event" , "Athletics contest" .)