Mark time (6)
I believe the answer is:
period
This clue is a double definition.
'mark' is the first definition.
(I know that period is a type of punctuation)
'time' is the second definition.
(I know that period is a type of geologic time)
(Other definitions for period that I've seen before include "Spell, epoch" , "Era, epoch" , "term" , "School session" , "Stretch - cycle" .)