Slack water at dusk (8)
I believe the answer is:
eventide
'dusk' is the definition.
(I've seen this before)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for eventide that I've seen before include "End of the day (poetic)" , "onset of dusk" , "The day's close (poetic)" , "some time later?" , "sunset" .)