Add BACK before the word described by the first clue to get the word described by the second clue. The answer will be the longer word.
-1.BACK + To talk over the internet = Banter
-2.BACK + Piece of wood = Pending work
-3.BACK + Part of the human body = Spinal column
-4.BACK + Divisions in a hospital = Reverse direction (plural)
-5.BACK + Antonym of down = Copy of the original
-6.BACK + Injure with a knife = Betray
-7.BACK + Fruit of the palm tree = To bring into effect retrospectively
-8.BACK + Fall = Part of a stage setting
-9.BACK + To lightly touch someone or something = Swimming technique
10.BACK + Nip = To talk ill of someone in their absence