It's mainly concerned with the circulation of a letter or two! (5)
I believe the answer is:
aorta
'two' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'it's mainly concerned with the circulation of a letter' is the wordplay.
'it's mainly' means to remove the last letter.
'concerned' becomes 'ate' (something concerning might eat someone up).
'with' means one lot of letters goes inside another.
'the circulation of' becomes 'or' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'a letter' becomes 'a'.
'ate' with its last letter taken away is 'at'.
'at' enclosing 'or' is 'aort'.
'aort'+'a'='AORTA'
'or' is the link.
Can you help me to learn more?
(Other definitions for aorta that I've seen before include "Large trunk artery" , "Main arterial vessel, carrying blood to the heart" , "Bloody channel" , "Major artery at left of heart" , "Trunk route" .)