Solver pursuing Boatman up marshy creek (5)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

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).