Worry about half of them covering athletics event (9)
I believe the answer is:
triathlon
'athletics event' is the definition.
(I've seen this before)
'worry about half of them covering' is the wordplay.
I cannot really understand how this works, but
'about' could be 'on' (as in a book 'on' a certain topic) and 'on' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for triathlon that I've seen before include "Athletics event" , "Swimming/cycling/running event" , "sports event" , "Triple athletic event" , "Contest involving swimming, running and cycling" .)