That looks like one of the 4 that hold the cluster and the binacle, I don’t think it’s one of the nuts that actually hold the instruments to the back of the cluster, you will soon know because the treads are different.
If you do have to dive in, and it’s not obvious which thread has lost its nut, take the clock out first and if still no access then remove the speaker grill, you may be able to reach both the threads for the binacle and the cluster.