First, let us clarify: Is it
a publishable sceneplay to be read by readers, the creative choice of that format for storytelling, the final product to be "consumed" by your audience
a tool for director to create actual play or a movie, a step, ingredient to obtain the final product which will be the actual play?
There is a significant difference between the two formats. The former must not bore the reader with descriptions, must not skimp on details, and still can contain flair and effects simply unobtainable in real play. The latter may ignore aesthetics and pacing of the descriptions in favor for factual adherence, skimp on non-essential details as to not stifle creativity of the scene designer, and must consider the scenes to be realistically playable within reasonable SFX budget.
In the first case, the answer is pretty much the same as for writing novels. Contain all that is needed to build the scene in imagination of the reader, make sure it's flavorful and not boring. Try to stick to moderate size, not too laconic and not too boringly long. You may leave some essential details for later too.
In the second case, keep it short. Keep all non-essential details out of it, and all essential details in. Skimp on artistic forms.
In your particular example, it seems you're aiming more towards the first form. The professional killer should have been introduced in depth in "Dramatis Personae" and only mentioned by title here. The placement of the clock should be non-essential. We are not informed about what the work is. These are details you should not hold back from the director, although you may keep them back from the reader. What you wrote works fine for screenplay-as-book, but not for screenplay-for-movie. If you want the latter, it should be more like:
INT. VICTIM’S BASEMENT - NIGHT
MURDERER is hard at work digging a grave in the patch of soil of the basement. The clock can be seen, reading 3:00 AM.
The character's been long established. The actor is informed what to do. Essential element of the scene (the clock) is established, but its location not enforced. Detail unrelated to the appearance of the scene (the victim is unsuspecting) is skipped.