Writer's movement finished, author's right significantly advanced? (8)
I believe the answer is:
Here is my best explanation:
'writer's movement' is the definition.
(I've seen this before)
'finished author's right significantly advanced?' is the subsidiary indication.
I cannot really understand how this works, but
'finished' could be 'up' ('up' can be similar in meaning to 'finished') and 'up' is present in the answer.
'right' could be 'ok' (I've seen this in other clues) and 'ok' is present in the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for upstroke that I've seen before include "Movement of pen or bow", "Writer's movement", "bit", "Upward line in writing" and "Bit of writing".)