Military vessel (7)
(Other definitions for warship that I've seen before include "Eg, destroyer, cruiser" , "It's armed at sea" , "Fighting vessel" , "Navy asset" , "frigate perhaps" .)
'military vessel' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for gunboat that I've seen before include "Not a bug (anag)" , "Military vessel" , "Coastal warship" , "Small military ship" , "Naval symbol of aggression" .)