Well-filled vessel (3-6)
'vessel' is the definition.
(tanker is a kind of vessel)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for tanker that I've seen before include "Fuel carrier" , "Big ship for transporting oil" , "'Ship, carrying crude oil say (6)'" , "Bulk oil carrier" , "Bulk-liquid lorry" .)
(Another definition for oil-tanker that I've seen is " Bulk fuel freighter".)