Service Yankee vehicle on way north (4)
I believe the answer is:
navy
'service' is the definition.
(I know that navy is a type of service)
'yankee vehicle on way north' is the wordplay.
'yankee' becomes 'Y' (phonetic alphabet: alpha, bravo, charlie etc.).
'vehicle' becomes 'van' (van is a kind of vehicle).
'on way north' is a reversal indicator (in a down clue, letters go up or 'north').
'y'+'van'='yvan'
'yvan' back-to-front is 'NAVY'.
(Other definitions for navy that I've seen before include "Dark shade of blue" , "Senior service" , "Fleet of ships" , "Fleet - blue" , "Colour; fighting force" .)