As thief may be, and trapeze artist occasionally? (6,2,3,3)
I believe the answer is:
caught in the act
'as thief may be' is the definition.
(a thief might be caught in the act of thieving)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for caught in the act that I've seen before include "Clearly guilty" , "as a trapeze artist may be" , "as acrobat might be" , "Obviously responsible" , "Discovered doing something wrong" .)