The most common and most likely answer to this crossword clue is the 5 plus 4 plus 3 plus 4 plus 3 plus 4 letter word THROW ONES HAT INTO THE RING
I've seen this crossword clue in The Times
T
H
R
O
W
O
N
E
S
H
A
T
I
N
T
O
T
H
E
R
I
N
G
More clues leading to the result THROW ONES HAT INTO THE RING
Buddy explains!

The straight part
We know that "Be a contestant" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case be a contestant = throw ones hat into the ring.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.