Briefly flash LED on low/high change
Try adding a parallel resistor to "bleed" the capacitor while the driver output is high:



The R1*C1 combination gives a flash duration of about 100 us. As long as the LED is bright enough, it doesn't matter how short this duration --- your eye will still see it. If you want a visibly longer duration pulse you may want to look into something like a one-shot instead of a simple RC circuit.

I chose the R2 value to give a 10 ms time constant for discharging. This will allow the cap to be essentially fully discharged in the 500 ms your clock output is high. 

Note this solution drives some current back in to the Vcc supply. There should be some other load on Vcc to consume this current, or you might cause problems with power supply stability. If there isn't any other load on Vcc, add an additional 10 kOhms from Vcc to ground.