Empty tin put in tank (6)
I believe the answer is:
vacant
'empty' is the definition.
('vacant' can be similar in meaning to 'empty')
'tin put in tank' is the wordplay.
'tin' becomes 'can' ('can' can be a synonym of 'tin').
'put in' is an insertion indicator.
'tank' becomes 'vat' (I've seen this before).
'can' inserted into 'vat' is 'VACANT'.
(Other definitions for vacant that I've seen before include "Empty, unoccupied" , "Unoccupied - unthinking" , "Not filled, job or house" , "Without an occupant or incumbent" , "Horse in a race" .)