The most common and most likely answer to this crossword clue is the 4 plus 5 letter word TIME TRIAL
I've seen this crossword clue in The Daily Telegraph
T
I
M
E
T
R
I
A
L
More clues leading to the result TIME TRIAL
Buddy explains!
The straight part
We know that "Watch one’s opponent in this event" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case watch one’s opponent in this event = time trial.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.