Clipping is an issue, its basically distortion but the baddest kind of distortion, because it cuts of the peaks of your sound. Meaning, everything above the clipping level or the "ceiling" as was described before gets chopped of. The mastering engineer cannot restore this missing sound. Try to place a limiter or master compressor onto your master channel, so that the peaks are not chopped off but rather smoothened out.
On a side note, I'm not sure if it is a good idea to get your tune so loud that it clips if you want to give it to a mastering engineer in the first place. He will make the tune as loud as it can be anyway but without the loss of sound information. I would recommend to leave about -4 to -6 db of headroom, so there will be plenty of dynamic to work with when mastering.