Bad actors, fraudulent top to bottom (4)
I believe the answer is:
hams
'bad actors' is the definition.
(as in a hammy actor)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for hams that I've seen before include "Poor actors" , "Sit on such" , "inept performers" , "Overacts (colloq.)" .)