Possible Clues:

• Aspen is common without upper-class
• Tree in the willow family
• Timber producer of high standing until found to be heartless
• London metropolitan borough which became part of Tower Hamlets in 1965
• Deciduous tree
• Union leader leaves well-liked part of London
• Willow's kin
• Dad left artist back in London area
• Deciduous tree: London district, former borough
• Uniform leaves in tree