Car crash south of Aleppo cleared arterial route (5)
I believe the answer is:
aorta
'arterial route' is the definition.
(the aorta is an artery)
'car crash south of aleppo cleared' is the wordplay.
'car crash' becomes 'RTA' (abbreviation for road traffic accident).
'south of' says to put letters next to each other (in a down clue, some letters go under others).
'cleared' means to remove the middle letters (the inside letters are cleared out).
'aleppo' with its middle removed is 'ao'.
'rta' after 'ao' is 'AORTA'.
(Other definitions for aorta that I've seen before include "Blood-carrier from a heart" , "Body's main artery" , "Large artery of the heart" , "Vital blood vessel" , "A rota (anag)" .)