The most common and most likely answer to this crossword clue is the 9 plus 5 plus 6 letter word BADMINTON HORSE TRIALS
I've seen this crossword clue in The Guardian
B
A
D
M
I
N
T
O
N
H
O
R
S
E
T
R
I
A
L
S
Buddy explains!
The straight part
We know that "sporting event" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sporting event = badminton horse trials.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.