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've seen this in other clues).
'can' placed inside 'vat' is 'VACANT'.
(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!" .)