Sinks, as the sun (4)
I believe the answer is:
sets
'sinks as the sun' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for sets that I've seen before include "Scenery etc. of 11 across productions" , "Units of play in tennis" , "Plants" , "Scenery etc. of dramatic productions" , "Solidifies" .)