Old liberal teacher withdraws evidence of Book 9 (2,6)
I believe the answer is:
ex libris
'evidence of book 9' is the definition.
The answer and definition can be both related to communication as well as being singular nouns.
Maybe you can see an association between them that I don't see?
'old liberal teacher withdraws' is the wordplay.
'old' becomes 'ex'.
'liberal' becomes 'lib'.
'teacher' becomes 'sir' (I've seen this before**).
'withdraws' says the letters should be written backwards.
'sir' reversed gives 'ris'.
'ex'+'lib'+'ris'='EX-LIBRIS'
(Other definitions for ex libris that I've seen before include "Bookplate inscription" , "Commemorative bookplate" .)