Relating to warships (5)
I believe the answer is:
naval
'relating to warships' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for naval that I've seen before include "Relating to marine force" , "Relates to nautical defence force" , "Relating to sea forces" , "Relating to the Senior Service" , "Of seaborne forces" .)