The most common and most likely answer to this crossword clue is the 6 plus 2 plus 3 plus 3 letter word CAUGHT IN THE ACT
I've seen this crossword clue in The Times
C
A
U
G
H
T
I
N
T
H
E
A
C
T
Buddy explains!
The straight part
We know that "As thief may be/trapeze artist occasionally" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case as thief may be/trapeze artist occasionally = caught in the act.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.