Reserved one volume for the editor (6)
I believe the answer is:
booked
'reserved' is the definition.
('book' can be a synonym of 'reserve')
'one volume for the editor' is the wordplay.
'one volume' becomes 'book' (book is a kind of volume).
'for' means one lot of letters go next to another.
'the editor' becomes 'ed' (abbreviation).
'book'+'ed'='BOOKED'
(Other definitions for booked that I've seen before include "Reserved in literary way?" , "Reserved, bought ticket in advance" , "Reserved, arranged say seat in advance" .)