Nautical officer on board Victory (4)
I believe the answer is:
mate
'nautical officer' is the definition.
(I've seen this in another clue)
'on board victory' is the wordplay.
'on' becomes 'at' (I've seen this in other clues).
'board' indicates putting letters inside.
'victory' becomes 'me' (I can't justify this - if you can you should believe this answer much more).
'at' put into 'me' is 'MATE'.
(Other definitions for mate that I've seen before include "Final check at chess" , "assistant" , "Move terminal for King" , "Perhaps the wife" , "provides solution to chess problem" .)