PITTSBURGH – The Chicago Cubs clinched a playoff spot under rookie manager David Ross, returning to the postseason after a one-year absence despite losing to the Pittsburgh Pirates 3-2 Tuesday night on a walk-off home run by Jacob Stallings.The NL Central-leading Cubs were assured a spot in the expanded playoff field when Philadelphia was swept in a doubleheader at Washington.The Cubs began the day with a 4 ½-game lead over St.