***SPOILER ALERT***

I killed the Deathclaw, received the Compton cleared message, then I went back into the building, did a Quicksave.

I tried killing everyone for giggles, was unable to. Reloaded the Quicksave.

Now the quest to clear Compton is back, the icon stays lit that shows where the Deathclaw WAS, but it is not there anymore.