Main artery forming part of that road heading north (5)
I believe the answer is:
aorta
'main artery' is the definition.
(I've seen this before)
'part of that road heading north' is the wordplay.
'part of' indicates a hidden word.
'heading north' says the letters should be written backwards.
'atroa' is hidden within 'that road'.
'atroa' backwards is 'AORTA'.
'forming' acts as a link.
(Other definitions for aorta that I've seen before include "sort of vessel" , "Main artery of body" , "Vital blood vessel" , "This carries blood from the heart" , "Main route from the heart" .)