Empty container placed inside another (6)
I believe the answer is:
vacant
'empty' is the definition.
(similar in meaning)
'container placed inside another' is the wordplay.
'container' becomes 'can' (can is a kind of container).
'placed inside' means one lot of letters goes inside another.
'another' becomes 'vat' (I can't justify this - if you can you should give a lot more credence to this answer).
'can' inserted inside 'vat' is 'VACANT'.
Can you help me to learn more?
(Other definitions for vacant that I've seen before include "Horse in a race" , "Of house, not occupied at present" , "available" , "Empty, unoccupied" , "as gents often are!" .)