Sink hole (5)
'sink hole' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for drain that I've seen before include "Make weak" , "To empty (of fluid)" , "Waste channel/pipe" , "Draw off U sewer" , "Draw off liquid" .)
(Other definitions for basin that I've seen before include "Canal harbour" , "dock, maybe" , "Area drained by a river" , "Round open dish for water" , "Stoup" .)