Exploited, improperly sued (4)
I believe the answer is:
used
'exploited' is the definition.
(synonyms)
'improperly sued' is the wordplay.
'improperly' indicates anagramming the letters (I've seen this in other clues).
'sued' anagrammed gives 'USED'.
(Other definitions for used that I've seen before include "worked unfairly" , "Accustomed, having been put into service" , "Utilised" , "Exploited group" , "Employed in task, or second-hand" .)