Rota negotiated with a vital supplier (5)
I believe the answer is:
aorta
'vital supplier' is the definition.
(vital artery)
'rota negotiated with a' is the wordplay.
'negotiated with' indicates anagramming the letters (negotiate can mean to organise or arrange).
'rota' anagrammed with 'a' makes 'AORTA'.
(Other definitions for aorta that I've seen before include "Vital blood vessel" , "Trunk route" , "sort of vessel" , "Heart's great artery" , "Body's main artery" .)