Traveller left in vessel — set out early (7)
I believe the answer is:
preplan
'set out early' is the definition.
'traveller left in vessel' is the wordplay.
'traveller' becomes 'rep' (travel company worker).
'left' becomes 'l' (common abbreviation).
'in' indicates putting letters inside.
'vessel' becomes 'pan' (I have seen 'Shallow vessel ' mean 'pan' so perhaps 'vessel' could also mean 'pan').
'rep'+'l'='repl'
'repl' inserted into 'pan' is 'PREPLAN'.
(Other definitions for preplan that I've seen before include "Plot in advance" , "In advance, design" .)