If I understand correctly, you have a clogged or partially clogged bathtub drain. I usually take a wet sock or handtowel and stuff it in the upper tub drain, run some water till it backs up and plunge away. Usually this clears all obstructions. If the clog is in the bathtub drain this should usually work.
Clear Any Clogged Drain | Plumbing | Plumbing, HVAC & Electrical | This Old House