Container inside another is free (6)
I believe the answer is:
vacant
'is free' is the definition.
(thesaurus)
'container inside another' is the wordplay.
'container' becomes 'can' (can is a kind of container).
'inside' means one lot of letters goes inside another.
'another' becomes 'vat' (I've seen this in other clues).
'can' put into 'vat' is 'VACANT'.
(Other definitions for vacant that I've seen before include "... Blank" , "Not filled, job or house" , "blankly incurious" , "Not occupied, perhaps apartment" , "thoughtless" .)