Related Clues:

• Pro __ (proportionally)
• Aspen, for one
• Ygdrasil or rowan
• Biological cavity
• Apple, plum or pear, e.g.
• Yggdrasil, e.g.
• It might be filled with ink
• Madrid maze-runner
• Almond, e.g.
• Koala's hangout
• Spider's pouch
• Genealogist's drawing
• Pre-Christmas buy
• See 57 Across
• See 38 Down