Empty tank container to be loaded (6)
I believe the answer is:
vacant
'empty' is the definition.
('vacant' can be similar in meaning to 'empty')
'tank container to be loaded' is the wordplay.
'tank' becomes 'vat' (I've seen this before).
'container' becomes 'can' (can is a kind of container**).
'to be loaded' means one lot of letters goes inside another.
'vat' enclosing 'can' is 'VACANT'.
(Other definitions for vacant that I've seen before include "Not filled, job or house" , "available" , "Of house, not occupied at present" , "blankly incurious" , "Empty, not occupied" .)