Criminals (6)
'criminals' is the definition.
(I know this)
This is all the clue.
(Other definitions for crooks that I've seen before include "Rustlers" , "taken in hand by bishops" , "More than one criminal" , "at 16 down [MAN UNITED]" , "Criminals and bishops' staves" .)
(Other definitions for felons that I've seen before include "Criminals, outlaws" , "Malefactors, criminals" , "Convicted criminals" .)