The surest solution is to remove the P-trap, flush out the clog, and reinstall the trap. You can also try clearing the clog from above. You might not even need the wrench in your toolbox. Just unscrew the ball-and-socket connection that holds the assembly together in the back of the sink, slide out the rod, and remove the pop-up stopper. The offending glob—or at least enough of it to get the drain flowing again—is likely to come out on the end of the stopper. Clean it off, replace and hand-tighten the its ball-and-socket nut.