***** NOT A TEACHER *****
Hello,
I think that we can parse it this way:
(You) = subject.
leave = verb.
it = object.
as = "in the way in which"
it = subject.
is = verb. ( = exists)
Thus, as you suggested, "as it is" is an adverbial clause modifying "leave."
Answers the question "How should I leave it?"
NOTES:
1. Some books refer to "as" in your sentence as a conjunction. Other books feel it is more accurate to call it a relative adverb.
2. Some people use the "incorrect" like in your sentence: "Leave it like it is."
3. Some people leave out the subject of the subordinate clause: "Hey! Leave it as is!"
James
References:
House and Harman, Descriptive English Grammar (1950).
Paul Roberts, Understanding Grammar (1954).