Tthe admiral initially drank rum in this vessel (7)
I believe the answer is:
tankard
'this vessel' is the definition.
(tankard is a kind of vessel)
'tthe admiral initially drank rum' is the wordplay.
'initially' indicates taking the first letters.
'rum' is an anagram indicator (rum can mean strange or unusual).
The first letters of 'tthe admiral' is 'ta'.
'drank' anagrammed gives 'nkard'.
'ta'+'nkard'='TANKARD'
'in' is the link.
(Other definitions for tankard that I've seen before include "Tall beer mug" , "Drinker's pot" , "Large drinking vessel" , "One-handled beer mug" , "A drinking mug with handle" .)