Without knowing what kind of finish is on them now it's difficult to recommend a finish. The most common clear finish would be a water based polyurethane however it shouldn't be used over another finish unless it's water based poly. Any oil based varnish or polyurethane is going to yellow. It's the nature of oil based finishes. Lacquer, if the existing finish is an oil based varnish then the solvents in it could wrinkle up and ruin the existing finish. With lacquer you have to be careful which one. The most common lacquer is known as a nitrocellulose lacquer which is made from cotton and is prone to yellow. There are acrylic lacquers though that would remain clear. A pre-catalyzed lacquer is one that will remain clear. The only thing you might do is find an inconspicuous place on the furniture and wet it with lacquer thinner or a drop of lacquer and see what happens.
As best as I can tell from the pictures is the finish that is on there now has yellowed already. It's likely to continue to yellow even if you put a clear finish over the top. The only real solution would be to refinish the furniture and go back with a non-yellowing finish. If you choose this route you better wait a while. Stripping furniture isn't a winter sport. No remover works well below 70 degrees and actually works better at 90 degrees.