A smooth flow late in the day (8)
I believe the answer is:
eventide
'late in the day' is the definition.
(eventide means the evening)
'a smooth flow' is the wordplay.
'a smooth' becomes 'even' (I've seen this before).
'flow' becomes 'tide' (tiding is a kind of flowing).
'even'+'tide'='EVENTIDE'
(Other definitions for eventide that I've seen before include "when it starts to get dark" , "End of the day (poetic)" , "late in the day?" , "End of the day to the poet" , "sunset" .)