Of the fleet (5)
I believe the answer is:
naval
'of the fleet' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for naval that I've seen before include "Relating to the Senior Service" , "Relating to marine force" , "Relates to nautical defence force" , "Relating to sea forces" , "To do with the marine branch of an army" .)