Diamonds seen by river in partial darkness (4)
I believe the answer is:
dusk
'partial darkness' is the definition.
(I've seen this in another clue)
'diamonds seen by river' is the wordplay.
'diamonds' becomes 'd' (abbreviation for the card suit diamonds).
'seen by' means one lot of letters go next to another.
'river' becomes 'usk' (Welsh river).
'd'+'usk'='DUSK'
'in' is the link.
(Other definitions for dusk that I've seen before include "Time towards the end of the day" , "Sundown" , "Period after sunset" , "Eventide" , "Time between twilight and night" .)