I did this crossword last week, and admit I wasn't too sure myself, apart from
Twitterboned does mean deformed hoof and
Twitter - Yak nervously
If you take E = Earth ( although Chambers doesn't give this abbreviation) and D - died ( which it does) this would leave BON - which could nob ( a 'turning' of butter) with backwards = over.
As I look at this answer - it does seem rather convoluted. But I'm sure twitter boned IS the answer.