A type, not the first to get on a vessel (5)
I believe the answer is:
aorta
'vessel' is the definition.
(blood vessel)
'a type not the first to get on a' is the wordplay.
'type not' becomes 'or' (I can't explain this - if you can you should give a lot more credence to this answer).
'the first to' becomes 't' (ost letter of 'to').
'get on' is a charade indicator (letters next to each other) (in a down clue, some letters go above others).
'a'+'or'+'t'+'a'='AORTA'
Can you help me to learn more?
(Other definitions for aorta that I've seen before include "Blood carrier" , "Large artery of the heart" , "vital transporter" , "Main artery of body" , "it's drawing blood!" .)