Twilight start to drive by Welsh river (4)
I believe the answer is:
dusk
'twilight' is the definition.
(synonyms)
'start to drive by welsh river' is the wordplay.
'start to' says to take the initial letters.
'by' means one lot of letters go next to another.
'welsh river' becomes 'usk' (I've seen this in another clue).
The first letter of 'drive' is 'd'.
'd'+'usk'='DUSK'
(Other definitions for dusk that I've seen before include "Time of day after sunset" , "Nightfall" , "Gloaming" , "Time between twilight and night" , "Time towards the end of the day" .)