Warning: Graphic imagery ahead!
If close allies Superman and Wonder Woman were forced to fight to the death, it's no contest: Diana would absolutely be able to kill Clark. The two unfortunately found out how a match-up would shake out after Superman was manipulated by Maxwell Lord's mind-control powers. Though the Man of Steel fought with a frightening tenacity, Wonder Woman proved she's ready and able to take him down if she ever has to.
Of course, both heroes have their own unique traits that make them quite amazing fighters. Thanks to his Kryptonian heritage, Clark has a veritable trove of powers that have made him one of the most capable heroes on the planet. By contrast, Diana's godly origins have enabled her to fight some of the most legendary monsters and otherworldly threats. But when pitted against each other, only one can come out on top.
Wonder Woman proved she does, in fact, have what it takes to bring down Superman in Wonder Woman #219 by Greg Rucka, Rags Morales, David Lopez, Tom Derenick, Georges Jeanty and Karl Kerschl. After Maxwell Lord implants a vision of Doomsday killing Lois Lane, Superman goes berserk and attempts to kill Wonder Woman. He shoots lasers at Diana, who effortlessly blocks his beams with her bracelets. Said bracelets also come in handy to help Wonder Woman immobilize Superman by clanging them right in his ears. She even attempts to distract Clark by calling upon birds to give her enough time to break Lord's control over the hero. Maxwell smugly comments that nothing will stop Superman, prompting Diana to throw her tiara at Clark, nicking his throat and finally getting him to stop his rampage.
The event is a bitter, bloody fight between friends, but it proved that in a worst-case scenario, Wonder Woman could kill Superman if she needed to. If it were any other hero, they'd likely be obliterated by an out-of-control Kryptonian, but Diana's resilient nature makes her one of the few people able to last in a fight with Clark. And it isn't just her raw power that gives Wonder Woman what she needs to turn the tide of the fight. After all, Diana is an Amazon, who are some of the best warriors in the DC Universe. She applies her training from her days on Themyscira and uses it to help her take advantage of Clark's weak points, like his hearing or his vulnerability to magic. Wonder Woman may not have strength to rival Superman, but she doesn't need it if she can fight smarter.
Many of the Greek legends Diana is familiar with often feature powerful foes felled by a clever hero. Much like Achilles, even a great warrior like Clark can be stopped by a simple cut. Though Wonder Woman and Superman remain close, the former has a trump card waiting for when she needs it.