Empty one container into another (6)
I believe the answer is:
vacant
'empty' is the definition.
(similar in meaning)
'one container into another' is the wordplay.
'one container' becomes 'can' (can is a kind of container).
'into' is an insertion indicator.
'another' becomes 'vat' (I've seen this in other clues).
'can' placed inside 'vat' is 'VACANT'.
(Other definitions for vacant that I've seen before include "available" , "Empty - stupid" , "Not occupied, perhaps apartment" , "Not filled, job or house" , "blankly incurious" .)