Clue: Predecessor of 1 down on both sides of the pond (6)
Answer: WILSON
Explanation: Definition: Predecessor of 1 down on both sides of the pond
Refers to Woodrow WILSON (who was a president of the US before Lyndon B Johnson) and Harold Wilson (who was a PM before Boris Jonhnson), *however "predeecessor" normally means the person immediately preceding the incumbent, so the predecessor to Boris Jonson is Theresa May, and the predecessor of Lyndon B Johnson was John F Kennedy.