Vessel not returning to part of Switzerland (6)
I believe the answer is:
canton
'to part of switzerland' is the definition.
(Swiss administrative subdivision)
'vessel not returning' is the wordplay.
'vessel' becomes 'can' (type of container).
'returning' shows that the letters should be reversed in order.
'not' in reverse letter order is 'ton'.
'can'+'ton'='CANTON'
(Other definitions for canton that I've seen before include "port in China" , "Political district of Switzerland" , "Administrative region in Switzerland" , "Ordinary" , "It cannot be in Switzerland" .)