Stupidly fire a gun inside the vessel (7)
I believe the answer is:
frigate
'the vessel' is the definition.
(frigate is a kind of vessel)
'stupidly fire a gun inside' is the wordplay.
'stupidly' indicates an anagram.
'a gun' becomes 'gat' (gat is a kind of gun).
'inside' is an insertion indicator.
'fire' anagrammed gives 'frie'.
'frie' enclosing 'gat' is 'FRIGATE'.
(Other definitions for frigate that I've seen before include "Ship" , "Naval escort vessel" , "US warship" , "Warship; -- bird" , "Type of warship" .)