Coming north, salesman with fish in carriage (7)
I believe the answer is:
sleeper
'carriage' is the definition.
(sleeper is a kind of carriage)
'coming north salesman with fish' is the wordplay.
'coming north' says the letters should be written in reverse.
'salesman' becomes 'rep' (I've seen this before).
'with' means one lot of letters go next to another.
'fish' becomes 'eels' (I've seen this before).
'rep'+'eels'='repeels'
'repeels' reversed gives 'SLEEPER'.
'in' is the link.
(Other definitions for sleeper that I've seen before include "provide on-line support?" , "Log laid under railway track, sounds somnolent" , "Wagon-lit" , "Rip Van Winkle" , "latent spy" .)