The most common and most likely answer to this crossword clue is the 4 plus 3 letter word MOSH PIT
I've seen this crossword clue in The Times
M
O
S
H
P
I
T
More clues leading to the result MOSH PIT
Buddy explains!
The straight part
We know that "part of rock arena" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case part of rock arena = mosh pit.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.