Vessel going West in trip for so long (8)
I believe the answer is:
farewell
'so long' is the definition.
(I've seen this before)
'vessel going west in trip' is the wordplay.
'vessel' becomes 'ewer' (ewer is a kind of vessel**).
'going west' is a reversal indicator.
'in' indicates putting letters inside.
'trip' becomes 'fall' (fall is a kind of trip).
'ewer' in reverse letter order is 'rewe'.
'rewe' inserted into 'fall' is 'FAREWELL'.
'for' is the link.
(Other definitions for farewell that I've seen before include "Parting word" , "Departure" , "Archaic goodbye or leave-taking" , "symphony" , "Say goodbye" .)