Mess left inside ship (7)
I believe the answer is:
clutter
'mess' is the definition.
(clutter is mess or disorganisation)
'left inside ship' is the wordplay.
'left' becomes 'l' (common abbreviation).
'inside' indicates putting letters inside.
'ship' becomes 'cutter' (type of ship).
'l' inserted into 'cutter' is 'CLUTTER'.
(Other definitions for clutter that I've seen before include "Untidy accumulation of objects" , "Things lying about untidily" , "Disorderly heap" , "Confused mess" , "Clog up" .)