Water moving smoothly in latter part of day? (8)
I believe the answer is:
eventide
'latter part of day?' is the definition.
I don't understand how the remainder of the clue works.
(Other definitions for eventide that I've seen before include "sunset" , "The day's close (poetic)" , "latter part of the day" , "late in the day?" , "End of the day (poetic)" .)