Employed sued criminal (4)
I believe the answer is:
used
'employed' is the definition.
('use' can be a synonym of 'employ')
'sued criminal' is the wordplay.
'criminal' indicates an anagram.
'sued' anagrammed gives 'USED'.
(Other definitions for used that I've seen before include "Accustomed - exploited" , "No longer new" , "Put to a purpose" , "Sued, having been employed" , "Took advantage of" .)