Deputy in vehicle heading northbound (3)
I believe the answer is:
sub
'deputy' is the definition.
(thesaurus)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for sub that I've seen before include "Advance payment" , "Warship; membership fee" , "Prefix meaning beneath" , "Type of sandwich" , "Underwater boat" .)