Possible Clues:

• What the postman should have where there's a cross dog
• Knight's attire
• What a postman might need where the dogs are fierce
• Perhaps postal uniform for night duty, by the sound of it
• Will this protect 1 across or a man of war?
• A postman's uniform, perhaps, for 12 across of old
• What the postman might need where there's a cross dog