Major artery in Longleat Road to the west (5)
I believe the answer is:
aorta
'major artery' is the definition.
(I've seen this in another clue)
'in longleat road to the west' is the wordplay.
'in' indicates the answer is hidden within the clue.
'to the west' is a reversal indicator (letters written right to left or 'east to west').
'atroa' is hidden within 'longleat road'.
'atroa' backwards is 'AORTA'.
(Other definitions for aorta that I've seen before include "blood flows from it" , "Major artery of the body" , "Large trunk artery" , "Great artery in body" , "This carries blood from the heart" .)