Main artery forming part of 'Great Road' to the north (5)
I believe the answer is:
aorta
'main artery' is the definition.
(largest artery)
'part of great road to the north' is the wordplay.
'part of' says the answer is hidden in the clue.
'to the north' is a reversal indicator (in a down clue, letters go up or 'northward').
'atroa' is hidden in the letters of 'great road'.
'atroa' backwards is 'AORTA'.
'forming' is the link.
(Other definitions for aorta that I've seen before include "Great artery in body" , "One leads from heart" , "Main route from the heart" , "Body's main artery" , "Tube" .)