A rota arranged to play a vital part in circulation (5)
I believe the answer is:
aorta
'play a vital part in circulation' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'a rota arranged' is the wordplay.
'arranged' indicates anagramming the letters (letters rearranged).
'rota' anagrammed gives 'orta'.
'a'+'orta'='AORTA'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for aorta that I've seen before include "This carries blood from the heart" , "Main artery from heart" , "blood flows from it" , "claret supplier" , "One leads from heart" .)