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