Possible Clues:

• Mix on the range
• Cross Norwegian king turned back on insect
• Swede turns on working party, becoming cross
• Meat hybrid
• Steak source
• Low-cholesterol steak source
• Grouse a lot, endlessly cross
• Second letter written before, essentially describing a line that's been crossed
• Following worker, perhaps, upset Scandinavian king is cross
• Mixed stock