Catch ---, apprehend in the act of wrongdoing (3-6)
I believe the answer is:
red-handed
(Other definitions for red-handed that I've seen before include "(Taken) in the act" , "(Caught) in the act of committing a crime" , "Catch someone thus in the act" , "In the act of crime - may be caught thus" , "Caught ... ......, apprehended in the act of crime" .)