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' is a charade indicator (letters next to each other) (in a down clue, some letters go under others).
'cleared' says to hollow out the word (remove centre letters) (the inside letters are cleared out).
'aleppo' with its centre removed is 'ao'.
'rta' put 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)" .)