Final messages needing money spent by Somerset city (9)
I believe the answer is:
'final messages' is the definition.
(messages said before leaving)
'money spent by somerset city' is the wordplay.
'money spent' becomes 'fare' (eg a train fare).
'by' means one lot of letters go next to another.
'somerset city' becomes 'wells' (Wells is a city in Somerset).
'needing' is the link.
(Other definitions for farewells that I've seen before include "They may be fond" , "spoken last words" .)