Plain that declaration hasn't finished (8)
I believe the answer is:
manifest
'plain' is the definition.
(I know that plain can be written as manifest)
'declaration hasn't finished' is the wordplay.
'declaration' becomes 'manifesto' (manifesto is a kind of declaration).
'hasn't finished' means to remove the last letter.
'manifesto' with its final letter removed is 'MANIFEST'.
'that' is the link.
(Other definitions for manifest that I've seen before include "Plain - cargo list" , "List of ship's cargo or passengers" , "list of passengers flying" , "Obvious, like ship's cargo list" , "Obvious to the eye" .)