Agree with slc's and upnover's advice.
The problem with clipping at this time of year is that spring coats are coming in under the long hair. If the horse in question is gray, appaloosa or otherwise light colored, it doesn't matter. If the horse is black, dark bay or chestnut, you're risking clipping off some of the spring coat as it's growing in, which means the horse will have a funky matte coat color. To explain further, pigment migrates to the end of the hair shaft; if you clip off the ends of the incoming coat, you'll lose some of the pretty new color. This is why show horses are clipped frequently and on a strict schedule, so they maintain some of the natural coat color.
If the issue is being able to put the horse in work without overheating and or taking hours to cool out, I would clip anyway. My horses have just barely started shedding, and won't have their full spring coats til April or May. A funky coat color for a month wouldn't stop me from clipping right now.