The most common and most likely answer to this crossword clue is the 9 letter word OPPONENTS
O
P
P
O
N
E
N
T
S
Buddy explains!
The straight part
We know that "Rivals" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case rivals = opponents.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.
What does OPPONENTS mean?
OPPONENTS noun- One who opposes another; one who works or takes a position against someone or something; one who attempts to stop the progress of someone or something.