Circulation unit (6)
I believe the answer is:
reader
'circulation unit' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for reader that I've seen before include "Person perusing a book or newspaper" , "One who studies - or his book" , "Lecturer; book user" , "Peruser of texts" , "University lecturer, the highest rank below a professor" .)