Empty metal container covered by tax (6)
I believe the answer is:
vacant
'empty' is the definition.
(similar in meaning)
'metal container covered by tax' is the wordplay.
'metal container' becomes 'can' (I've seen this in another clue).
'covered by' is an insertion indicator.
'tax' becomes 'vat' (UK Value Added Tax).
'can' placed inside 'vat' is 'VACANT'.
(Other definitions for vacant that I've seen before include "Empty - stupid" , "Horse in a race" , "Unoccupied - unthinking" , "Empty, unoccupied" , "Not filled, job or house" .)