Highlight French writer's grave, for instance (6)
I believe the answer is:
accent
This clue is a double definition.
'highlight' is the first definition.
(I've seen this in another clue)
'french writer's grave for instance' is the second definition.
(grave is an type of accent used in written French)
(Other definitions for accent that I've seen before include "Distinctive manner of speaking" , "eg Acute" , "Stress - manner of speech" , "pitch" , "Here's, maybe, a grave" .)