Mess left aboard sailing vessel (7)
I believe the answer is:
clutter
'mess' is the definition.
(clutter is mess or disorganisation)
'left aboard sailing vessel' is the wordplay.
'left' becomes 'l' (common abbreviation).
'aboard' is an insertion indicator.
'sailing vessel' becomes 'cutter' (historical type of ship).
'l' placed within 'cutter' is 'CLUTTER'.
(Other definitions for clutter that I've seen before include "Confusion" , "Disorderly heap" , "Jumble" , "Clog up" , "Disorderly accumulation" .)