Ordinary accident holds up advanced vessel (5)
I believe the answer is:
aorta
'vessel' is the definition.
(blood vessel)
'ordinary accident holds up advanced' is the wordplay.
'ordinary' becomes 'O' (abbreviation eg in O-Level, Ordinary Level).
'accident' becomes 'rta' (I can't explain this - if you can you should believe this answer much more).
'holds up' means one lot of letters go next to another (in a down clue, some letters go beneath others).
'advanced' becomes 'a' (abbreviation).
'o'+'rta'='orta'
'orta' after 'a' is 'AORTA'.
Can you help me to learn more?
(Other definitions for aorta that I've seen before include "Main arterial vessel, carrying blood to the heart" , "Bloody channel" , "Vital blood vessel" , "Large trunk artery" , "Major blood vessel" .)