Beset by storm, left working vessel (7)
I believe the answer is:
galleon
'vessel' is the definition.
(type of ship)
'beset by storm left working' is the wordplay.
'beset by' means one lot of letters goes inside another ('beset' could historically mean 'cover').
'storm' becomes 'gale' (a gale is a windy storm).
'left' becomes 'l' (common abbreviation).
'working' becomes 'on' (on can mean working or activated).
'gale' going around 'l' is 'galle'.
'galle'+'on'='GALLEON'
(Other definitions for galleon that I've seen before include "Large Spanish sailing ship of yore" , "Trader" , "Historical warship with three or more masts" , "All gone (anag.)" , "One might sink" .)