Volume in electronic speech, not constant (7)
I believe the answer is:
edition
'volume' is the definition.
(edition can mean a volume or issue)
'electronic speech not constant' is the wordplay.
'electronic' becomes 'e' (as in e-mail or e-books).
'speech' becomes 'diction' (I've seen this in other clues).
'not' indicates named letters should be taken away.
'constant' becomes 'c' (mathematical abbreviation).
'diction' with 'c' removed is 'dition'.
'e'+'dition'='EDITION'
'in' is the link.
(Other definitions for edition that I've seen before include "Copies of book printed from one set of type" , "Particular version of a book, etc" , "(Copy from) single print run" , "single programme" , "Particular form of a book" .)