If the leaf turning yellow was preceded by irregular tan spots then it was probably caused by over watering or execessive rainfall. tomatos don't like wet feet... they should be both well watered and well drained. If the above wasn't encountered, then inspect the undersides of the leaves looking for small reddish to brownish mites. They may look like small pencil dot sized spots, but watch them for a moment to see if they move. If they do, spray the plant with a recommended mite lkiller... there are many here in the U.S. (mites and sprays)... If the plant dies, cut the stem near the ground and see if it has either a brown or black ring... this indicates blight of various kinds and you'll either want to treat the soil over winter or use a different area for planting next year.
If your plant recovers enough to set fruit, the resulting tomatos will be safe to eat... Best of luck!