Sinks out of sight, as the sun (4)
I believe the answer is:
sets
'sinks out of sight 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 "Plants" , "Places" , "Groups of things of the same kind" , "Hardens" , "Coagulates" .)