Possible Clues:

• Collective total - a great egg (anag)
• In sum there's a horse about the end of 28 across
• Something like a horse the boy ate as a whole
• Sum of all parts
• Put together
• A good boy devoured the whole lot
• As a whole, that's a whole horse the boy made a meal of
• The whole of 15 across about the way in
• Collective scores
• Solid rock a boy found in northern street