Mess is large in naval vessel (7)
I believe the answer is:
clutter
'mess' is the definition.
(clutter is mess or disorganisation)
'large in naval vessel' is the wordplay.
I cannot quite understand how this works, but
'large' could be 'l' (eg in clothes sizes) and 'l' is found within the answer.
'vessel' could be 'cutter' (cutter is a kind of vessel) and 'cutter' is found in the leftover letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
'is' acts as a link.
Can you help me to learn more?
(Other definitions for clutter that I've seen before include "Confused multitude of things" , "Untidy lot of objects in room" , "Untidy mess" , "Things lying about untidily" , "Confusion" .)