Go with a letter to Barnaby (6)
I believe the answer is:
trudge
'go' is the definition.
(trudging is a kind of going)
'with a letter to barnaby' is the wordplay.
'with' says to put letters next to each other.
'a letter' becomes 't' ().
'to barnaby' becomes 'rudge' (Barnaby Rudge is a Dickens novel).
't'+'rudge' is 'TRUDGE'.
(Other definitions for trudge that I've seen before include "tramp" , "Plod doggedly" , "Traipse" , "Walk with slow, heavy steps" , "Walk heavily, as if through mud" .)