Trunk route going back in Longleat Road (5)
I believe the answer is:
aorta
'trunk route' is the definition.
(I've seen this before)
'going back in longleat road' is the wordplay.
'going back' shows that the letters should be reversed in order.
'in' indicates a hidden word.
'atroa' can be found hidden inside 'longleat road'.
'atroa' backwards is 'AORTA'.
(Other definitions for aorta that I've seen before include "Main route from the heart" , "Major artery of the body" , "Blood carrier" , "Main artery of body" , "Trunk route" .)