What adapter would power device that runs on 3 D batteries?
There are a few possible problems.


Your power supply could be broken. Check it with a voltmeter. Jiggle the wires and make sure there's not an intermittent break somewhere.
Your power supply is actually putting out AC. Some wall warts are AC/AC. Does it say it puts out DC?
The batteries may be in parallel instead of in series, in which case you've already overvoltaged the clock's circuitry and may have destroyed it. Does it still work when you put batteries in?
Your power supply may not supply enough current to run the clock. That seems unlikely; clocks typically draw very little current. But you can check by seeing if the voltage on the output of your power supply collapses when you connect it to the clock.
You could have your polarity backwards. Make sure you've connected positive to positive, and negative to negative. Again, use a voltmeter.
You could have a bad connection. How are you connecting the adapter to the clock?