noun (n)

• the distance by which one thing clears another; the space between them(noun.attribute)

  source: wordnet30
• vertical space available to allow easy passage under something(noun.quantity)

  Synonym:
  headroom, headway
  

  source: wordnet30
• permission to proceed(noun.act)

  Example:
  The plane was given clearance to land.
  

  source: wordnet30
• The act of clearing; as, to make a thorough clearance.(noun)

  source: webster1913