Clear as a document (8)
I believe the answer is:
Here is my best explanation:
I believe this is a double definition.
'clear' is the first definition.
('manifest' can be a synonym of 'clear')
'a document' is the second definition.
(manifest is a kind of document)
'as' acts as a link.
(Other definitions for manifest that I've seen before include "It's obvious", "(Of ghost) appear", "List of ship's cargo or passengers", "Obvious, like ship's cargo list", "Evident", "Express", "list of passengers flying", "Display", "Easily noticed - cargo list" and "show".)