Go under (4)
'go under' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for sink that I've seen before include "Subside" , "Washing-up place" , "Descend to a lower level" , "Slump" , "Pocket (billiards)" .)
(Other definitions for stopover that I've seen before include "Break in a journey" , "Overnight stay" , "Stay (of just one night?)" , "that hotel can provide?" , "bed and breakfast?" .)