Plod along with a letter to Barnaby (6)
I believe the answer is:
trudge
'plod along' is the definition.
(thesaurus)
'with a letter to barnaby' is the wordplay.
'with' is a charade indicator (letters next to each other).
'a letter' becomes 't' ().
'to barnaby' becomes 'rudge' (barnaby rudge is a dickens novel).
't' put next to 'rudge' is 'TRUDGE'.
(Other definitions for trudge that I've seen before include "Walk with slow, heavy steps" , "Walk wearily" , "Traipse" , "Plod wearily" , "Footslog" .)