The judges scored it 29-26, 29-26, 29-28 for the winner of Season 15 of "The Ultimate Fighter."
Chiesa (12-2) used judo throws and sweeps to take Clarke down in dominating the first two rounds. Clarke, a Saskatoon native who fights out of Edmonton, kept the bout standing in the third round but it was too little, too late.
Clarke (11-3) had won his last two UFC fights but was the underdog against Chiesa. He had not fought for more than 10 months due to an elbow injury.
Chad (Money) Mendes, ranked No. 1 among featherweight contenders, used his power to stop No. 4 Ricardo (The Bully) Lamas in the first round of the main event at the Patriot Center.
Mendes (17-2) hurt Lamas with a blow to the head two minutes in and punished him until the fight was stopped at the 2:45 mark.
It was just the second loss in eight fights for Lamas (15-4) since dropping down to 145 pounds.
(Raging) Al Iaquinta, ranked 15th among lightweight contenders, rallied to beat (29-28, 27-30, 29-28) No. 14 Jorge (Gamebred) Masvidal in the co-main event. Masvidal left the cage in disbelief at the scoring.
The crowd greeted the decision with boos.
"Are you guys booing me?" Iaquinta, his face leaking blood, said in his post-fight interview in the cage. "You better not boo me, I fought my ass off."
He dropped several F-bombs on the crowd before leaving.
Masvidal (28-9) cut Iaquinta below the right eye after knocking him down in a ferocious finish to the first round.
Masvidal's takedowns defence and crisp striking dominated early although Iaquinta (12-3-1) kept coming and punished Masvidal with leg kicks.
Both men came into the bout on three-fight win streaks.