Possible Clues:

• '...in ___ tree'
• '...partridge in ___ tree'
• '... partridge in ___ tree'
• '... partridge in tree'
• '...and a partridge in ___ tree'
• 21-Across's 'Three Pieces in the Shape of ___'
• '...partridge in -- tree'
• ". . . partridge in ___ tree"
• '... in ___ tree'
• ''. . . in ___ tree''