Volume is an issue (7)
I believe the answer is:
edition
I believe this clue is a double definition.
'volume' is the first definition.
(edition can mean a volume or issue)
'an issue' is the second definition.
(edition is a kind of issue)
'is' acts as a link.
(Other definitions for edition that I've seen before include "Newspaper issue" , "Number of copies printed at one time" , "copy of work" , "Volume" , "One version of a published text" .)