Summers here get pretty hot, too, the same temperatures you mention, and I boarded in a stable with several drafts that are more used to chilly winters than to summer heat. None were clipped, some of them were older, some of the - young. They were provided with a run-in and the pastures, in which were they were turned out 24/7, had several shaded areas with a number of large trees. They had free access to water and salt licks. And all did absolutely well, no problems at all, although we gave them closer supervision during the hottest days.
By the way, proper, healthy coat helps termoregulation also during heat, not only freezing temperatures. I'd say - don't clip .
I have come a long way, to surrender my shadow to the shadow of my horse.