Senior officer’s first sailing-vessel — expensive, they say (9)
I believe the answer is:
brigadier
'senior officer's' is the definition.
(thesaurus)
I don't understand how the rest of the clue works.
(Other definitions for brigadier that I've seen before include "Rank above colonel" , "Army rank" , "Officer in charge of army unit, part of division" , "High-ranking army officer" , "Military leader" .)