I came across this sentence in a conversation book. It is by itself alone, so It doesn’t have context around it. The sentence is ”Please clean up the room after you.”
I think it could mean after you’ve done with the room or behind you.
I can see why this would be tricky for a learner to parse. The key is knowing that clean up after is a phrasal verb.
Cambridge defines it as:
clean up after sb
phrasal verbto remove dirt or problems that someone has made : I'm fed up with cleaning up after you all the time.
clean up after someone
phrasal verbto clean a place after someone has made it dirty or messy : Residents have been told to clean up after their dogs.
and M-W says:
clean up after
idiomto make a place clean after it has been left dirty or messy by (someone) : His mother is always cleaning up after him. You should learn to clean up after yourself.
As for the sentence in the book, I think I would word it slightly differently:
Please clean up the room after yourself.
That’s how I’ve usually heard the phrase used reflexively, when “you” has made the mess and “you” is expected to clean it.