I don't have the book immediately to hand to check the context, but it sounds to me like the description of a sheep disease (cloggets) - symptoms being a trembling of the "greebs" - presumably a PTerry invention - in female sheep (hoggets) - possibly related to Licky End - normally only found in pregnant sheep, but Gaspode suffers from it!

Given the dragon diseases that PTerry has come up with, this disease sounds fairly straightforward.

Oh, and the bird is a grebe, not a greeb.