Device ultimately dividing measure of liquid in vessel (7)
I believe the answer is:
galleon
'vessel' is the definition.
(type of ship)
'device ultimately dividing measure of liquid' is the wordplay.
'ultimately' indicates one should take the final letters.
'dividing' is an insertion indicator (inserted letters divide the word).
'measure of liquid' becomes 'gallon'.
The final letter of 'device' is 'e'.
'e' placed into 'gallon' is 'GALLEON'.
'in' acts as a link.
(Other definitions for galleon that I've seen before include "All gone (anag.)" , "Large Spanish sailing ship of yore" , "Old Spanish ship" , "Ship of the line" , "Old Spanish war-ship" .)