Possible Clues:

• Length unit, to 4-Across
• Mike chopped tree for length
• Ran into heads of regulatory enforcement from The Yard, for example?
• Initially massive tree chopped for length
• Standardised unit
• Unit found in Somme trenches
• It's about a yard, in England
• One of 50,460 in the Chunnel
• Unit of measure
• International unit of length