Replacement transport heading north (3)
I believe the answer is:
sub
'replacement' is the definition.
(substitute)
'transport heading north' is the wordplay.
'transport' becomes 'bus' (bus is a kind of transport).
'heading north' shows that the letters should be reversed in order.
'bus' written backwards gives 'SUB'.
(Other definitions for sub that I've seen before include "Prefix meaning beneath" , "Warship; membership fee" , "Underwater boat" , "Underwater vessel" , "Prefix meaning under or inferior" .)