Empty vessel filling another? (6)
I believe the answer is:
vacant
'empty' is the definition.
(similar in meaning)
'vessel filling another?' is the wordplay.
'vessel' becomes 'can' (type of container).
'filling' means one lot of letters goes inside another.
'another?' becomes 'vat' (I've seen this in other clues).
'can' inserted inside 'vat' is 'VACANT'.
(Other definitions for vacant that I've seen before include "Unoccupied - unthinking" , "Not occupied, perhaps apartment" , "available" , "Not filled, job or house" , "thoughtless" .)