... empty container inside another (6)
I believe the answer is:
vacant
'empty' is the definition.
('vacant' can be similar in meaning to 'empty')
'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 "thoughtless" , "Horse in a race" , "as gents often are!" , "Not filled, job or house" , "Unoccupied - unthinking" .)