Manuscript never ends: an old writer unfinished still (8)
I believe the answer is:
tranquil
'still' is the definition.
(I know that still can be written as tranquil)
'manuscript never ends an old writer unfinished' is the wordplay.
'ends' suggests the final letters.
'old writer' becomes 'quill' (historical writing implement).
'unfinished' means to remove the last letter.
The final letters of 'manuscript never' are 'tr'.
'quill' with its last letter taken away is 'quil'.
'tr'+'an'+'quil'='TRANQUIL'
(Other definitions for tranquil that I've seen before include "not making waves" , "Serenely quiet or peaceful" , "Peaceful and serene" , "Calm and undisturbed, at peace" , "Calm, placid, still" .)