Why When You Import A Clipping-Free Lossless Format & Export It As Lossy Format It Creates Clipping?
MP3 is an inexact format. If the peak level before encoding is close to 0 dB, then the peak level after encoding may be a little over 0 dB. This is particularly the case if the audio has been heavily limited or compressed. The actual amount of clipping caused by this effect is usually just isolated samples, though there may be a lot of them, so the clipping can look bad while not actually being audible.
To avoid the issue completely you would need to allow a bit of headroom before encoding (normalize the track a bit below 0 dB before exporting).
And… The MP3 itself (probably) isn’t clipped… MP3 can go over 0dB without clipping. Audacity can also go over 0dB and it shows potential clipping.
However, you will clip your digital-to-analog converter if you play it back at full-digital volume or if you make a WAV or audio CD from the over 0dB MP3. But, I’ve never heard of a case where the clipping caused by MP3 compression was audible. If you hear an MP3 compression artifact, it’s usually something else.