"It's not the funnest part of your job description being down 5-0 and saying, 'All right, I just got to get through all these innings,'" Clement said. "To let this one get out of hand was disappointing.
"I just didn't execute the pitches the way I needed to, and that's inexcusable."
The loss cut Boston's lead in the American League East to 2 1/2 games over the Yankees, who dismantled the Devil Rays, 17-3, on Tuesday.
Clement (13-5) yielded six runs on six hits in six innings. The six earned runs were the most the right-hander has allowed since giving up the same amount on Aug. 4 against the Royals.
After opening with a 1-2-3 first inning, Clement surrendered five runs in the second frame. One night earlier, the Jays put together a five-run seventh inning, but Boston recovered to win in 11 innings.
"[Clement] gathered himself and pitched pretty well [after the second inning]," Red Sox manager Terry Francona said. "Once you give up five, if they peck away and get one here and get one there, it makes it tougher to win."
In the second inning, Clement issued a leadoff walk to Corey Koskie, who moved to third base on a double by Shea Hillenbrand. After striking out Eric Hinske, Clement gave up a double to Toronto catcher Gregg Zaun -- giving the Blue Jays the early lead.
"I had [Zaun] in that situation. I had some good pitches to get ahead of him, and I didn't finish him," Clement said. "I didn't finish him with a good pitch, whether it be a groundout, a popout, whatever it may be -- and he was able to get the ball in the air.
"To me, that's the at-bat that if you get him out there and there's one run in, it's no big deal," he added. "But for him to get a double there keeps their rally going."
Zaun scored on a single by Aaron Hill, and then Gabe Gross put the final stamp on Clement's brief lapse. Gross pulled a pitch from Clement into the right-field bullpen.
"That kind of punctuates the inning," Francona said. "We were hanging in there -- he's hanging in there -- and all of a sudden, the ball leaves the ballpark and it makes a couple-run inning all of a sudden be a big-run inning."
After the home run, Clement didn't allow a hit to the next 10 batters he faced. Other than the second inning, the righty allowed just one run on two hits.
Francona called reliever Keith Foulke into the game in the seventh inning. It was the first time Foulke worked in consecutive games since rejoining the team after having surgery on his left knee.
Foulke and Chad Harville combined to allow three more runs in two innings.
By the time Boston's offense got rolling against Toronto left-hander Scott Downs, the damage had already been done.
Downs (3-3) gave up three runs on six hits in 5 1/3 innings to pick up his first win of September. Other than allowing the home run to Ortiz, the left-hander gave up an infield single by Gabe Kapler in the fifth inning and a sacrifice fly to Kevin Millar in the sixth.
Ortiz's shot was his 41st of the season, tying him with New York's Alex Rodriguez for the American League lead. It also was his third 400-plus foot solo homer in the last two games and the 37th home run he's hit this year while serving as Boston's designated hitter. That tied the single-season mark by a DH set in 2000 by Seattle's Edgar Martinez.
"I was pounding the zone for the most part and keeping the ball down," Downs said. "I got the curveball to Ortiz -- he's just a strong guy. It was a pretty good pitch, I thought, but that happens. I'm glad it was a 5-1 game instead of a 5-4 game."