Dip below the horizon (3)
I believe the answer is:
set
'dip below the horizon' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for set that I've seen before include "Where they're filming" , "Become solid like jelly" , "Group; unchanging" , "(Of the sun) drop below the horizon" , "group usually found together" .)