Clue: Twice Bill and I get together in a gum tree (6)
Answer: ACACIA
Explanation: Twice Bill and I get together in a gum tree (6)
A charade of AC ('bill') plus AC ('twice') plus 'I' plus (ignoring 'in') 'a'. The term 'gum tree' is not exact, but generally refers to various eucalypts, but the gum arabic tree is an acacia, so that the "definition" is at best an indication by example.