Hi Martyn
There are a few options
Slight clipping can be dealt with in a number of ways, but you probably can't remove the distortion, so you can try and mask it by adding more vocals at the same time, making another vocal line the primary one, you could add some grit it using a mic preamp plugin something like the Waves Scheps 73, or even a light use of a distortion pedal plugin, or reamping, or simply pushing the track up so all of it clips (but this will sound awful - but George Harrison used this)
Tinny can be dealt with by EQ'ing, remove som of the Hi might be enough.
Sharp can be fixed to a degree using Celemony Melodyne, but it won't do miracles, and you need to watch your buffer settings if you want to avoid distortion of the processed notes. Minor movements can be completely fixed.
Trust your ears, and alot depends on what other instruments/vocals are present in the song, and how many concurrent elements are playing.
Hope this helps
Rich