Clear as a document (8)
I believe the answer is:
Here is my best explanation:
This clue 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' is the link.
(Other definitions for manifest that I've seen before include "Cargo list - it's quite clear", "Obvious, cargo listing", "List of a ship's contents", "List of ship's cargo or passengers", "Become apparent", "The ship's cargo list is obvious", "Easily noticed - cargo list", "Apparent", "Conspicuous; cargo list" and "Ship's document; plain".)