Adventitia is a 10 letter word for outermost covering of an organ.
Organ
Organ
Cap
mat
quilt
Film
blanket
BARE
newspaper
lung
If you add one letter to the word "organ," it becomes "organ" + "i" = "organi," which isn't a valid word. However, if we consider the context of the question, adding a letter to "organ" can form the word "organs," pluralizing it to refer to multiple body organs. Alternatively, if you're looking for a specific organ like "heart" and add "h," it becomes "heart" + "h" = "hearth."
Wrap is a covering. Walled is an enclosure.