The most common and most likely answer to this crossword clue is the 12 letter word CHAMPIONSHIP
I've seen this crossword clue in The Sunday Telegraph
C
H
A
M
P
I
O
N
S
H
I
P
More clues leading to the result CHAMPIONSHIP
Buddy explains!

The straight part
We know that "competition" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case competition = championship.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 CHAMPIONSHIP mean?
CHAMPIONSHIP noun- A competition to determine a champion, especially the final of a series of competitions.
- The position of champion, or winner.
- Defense or support of some cause.