Clue: Solver pursuing Boatman up marshy creek (5)
Answer: BAYOU
Explanation: Definition: marshy creek
 YOU(as the setter calls the solver, using the second-person pronoun) **placed below**(pursuing, in a down clue) **reversal of**(… up, in a down clue) AB(abbrev. for for "able bodied seaman"/a boatman/one working on a boat).