End of day or start of day over river (4)
I believe the answer is:
dusk
'end of day' is the definition.
'start of day over river' is the wordplay.
'start of' says to take the initial letters.
'over' says to put letters next to each other.
'river' becomes 'usk' (Welsh river**).
The initial letter of 'day' is 'd'.
'd'+'usk'='DUSK'
'or' is the link.
(Other definitions for dusk that I've seen before include "Gloaming" , "Evening light" , "Dark twilight" , "The darker stage of twilight" , "Time between twilight and night" .)