The drowning pool

"A nervous and neurotic oil heiress, Iris Devereaux, lands her ex-flame Harper in hot water when she hires him to get to the bottom of an anonymous blackmail threat."@en
"Feature film from the USA. Mystery thriller. Private detective helps a friend caught up in a blackmail incident."
"Private eye Lew Harper is in deep this time. Hired by an old flame to unravel a seemingly routine blackmail case, he's so far down he may never come up for air. Paul Newman returns as the quick-witted detective he first played nine years before in "Harper." Joanne Woodward plays the New Orleans oil heiress who turns to Harper for help. Young Melanie Griffith is her kittenish daughter. And Tony Francioosa, Coral Browne, Andy Robinson, Murray Hamilton and more keep "The Drowning Pool's" intrigue as thick as gumbo."
"Private investigator Lew Harper is hired by a nervous New Orleans oil heiress, who also happens to be an old flame, to help her solve a problem that appears to be a routine blackmail threat but soon gets him involved with extortion, torture, murder and revenge."@en
"Paul Newman returns as the quick-witted detective he first played nine years before in Harper, who's in deep this time. Hired by an old flame to unravel a seemingly routine blackmail case, he's so far down he may never come up for air."@en
"Private eye Lew Harper investigates an anonymous blackmail threat against an oil heiress and finds almost more trouble than he can handle."