Possible Clues:

• Height (above ground)
• That'll tell us what you've been getting up to!
• Loftiness of US city overthrown at start of play
• Dilute at 13's level of verticality
• Poet's left out last part about bird on lake in a high place
• The height of the twisted tail of the duet
• Having a strange tilt, perhaps due to the height
• Height reached by bird replacing one left in advert
• Measure of how high you are
• Height from position seeing lake for the first time