Empty container covered by tax (6)
I believe the answer is:
vacant
'empty' is the definition.
('vacant' can be similar in meaning to 'empty')
'container covered by tax' is the wordplay.
'container' becomes 'can' (can is a kind of container).
'covered by' means one lot of letters goes inside another.
'tax' becomes 'vat' (British sales tax).
'can' put within 'vat' is 'VACANT'.
(Other definitions for vacant that I've seen before include "Not filled, job or house" , "thoughtless" , "available" , "as gents often are!" , "Unoccupied - unthinking" .)