Possible Clues:

• Horrid glances from Charles Grodin?
• Rearrangement of a word's letters to form new word(s)
• Elgar's regal composition
• A horse and farm animal is a mix of characters
• Hedonism for Sondheim
• Letters are needed to make a horse so sheepish at last
• A couple of beasts uniting could present a problem
• Aileen and Elaine, for instance
• 'Enraged' is of 'angered'
• 5 down is one