Possible Clues:

• Tsar crossing European border or boundary
• Bound to find something frozen in safe
• See 21-Down
• Fairy came across queen on edge
• Enclosing boundary
• Border safe, having been cleared of frost?
• Boy collects mud going round the boundary
• Chieftain turned up inside safe boundary
• Boundary of enclosure
• Taken all round, this might give on the degree of fairness, perhaps