Volume we may consult to be sure (9,4)
I believe the answer is:
reference book
'volume' is the definition.
The definition and answer can be both related to communication as well as being singular nouns.
Maybe there's a link between them I don't understand?
'we may consult to be sure' is the wordplay.
I cannot really understand how this works, but
'consult' could be 'refer' (I've seen this before) and 'refer' is found within the answer.
'sure' could be 'ok' (both are affirmative answers) and 'ok' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for reference book that I've seen before include "Encyclopedia, say" , "Volume providing information" , "Where to find information" , "Authoritative non-fictional tome for consultation" , "A source of facts" .)