It's of main importance where circulation's concerned (5)
I believe the answer is:
aorta
'where circulation's concerned' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'it's of main importance' is the wordplay.
I cannot quite see how this works, but
'of' could be 'o' and 'o' is found in the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found in the answer.
The remaining letters 'ara' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for aorta that I've seen before include "One leads from heart" , "Blood-carrier from a heart" , "Tube" , "Main artery from heart" , "Large trunk artery" .)