Empty tin in barrel (6)
I believe the answer is:
vacant
'empty' is the definition.
('vacant' can be similar in meaning to 'empty')
'tin in barrel' is the wordplay.
'tin' becomes 'can' ('can' can be a synonym of 'tin').
'in' indicates putting letters inside.
'barrel' becomes 'vat' (I've seen this before).
'can' put inside 'vat' is 'VACANT'.
(Other definitions for vacant that I've seen before include "Empty - stupid" , "available" , "Not filled, job or house" , "as gents often are!" , "Without an occupant or incumbent" .)