Emptied (7)
(Other definitions for drained that I've seen before include "empty" , "Emptied of liquid" , "Exhausted" , "spent" , "Deprived of strength" .)
'emptied' is the definition.
('vacate' can be a synonym of 'empty')
This is all the clue.
(Another definition for vacated that I've seen is " Quitted".)