Probably just a bulb on its way out. There are two (or three) that handle dash illumination, so the absence of one bulb will darken at least one of the gauges in the cluster.

You can verify bulb operation by rapping against the dash near the cluster...if the clock illumination returns, a bad bulb is the suspect.

The bulbs are 2W units. These are a bit difficult to find at auto parts stores, but the dealer should have them for around $4-$6.

The gauge cluster must be removed from the dash in order to gain access to the bulbs.

I don't know if you can just pull the cluster out, as you have a 99. My 93 300E and 95 SL500 clusters just pull out (snug fit), but my 99 ML320 cluster is bolted to the dash.

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car