Killer Instinct (1991)

An up and coming lawyer gets turned over for a case he'd been working on for a long while. The D.A.'s niece then decides to help the young lawyer by first blackmailing, then eventually plotting the murder of the D.A., all the while seducing the young attorney. It then becomes the attorney job to cover up the murder he believes he committed, while finding out the truth about the D.A.'s "niece".