It sustains teachers with old poetry books (9)
I believe the answer is:
nutriment
'it sustains' is the definition.
(I've seen this before)
'teachers with old poetry books' is the wordplay.
'teachers' becomes 'nut' (National Union of Teachers).
'with' is a charade indicator (letters next to each other).
'old poetry' becomes 'rime' (I have seen 'dated poetry' mean 'rime' so perhaps 'poetry' could also mean 'rime'. I am not sure about the 'old' bit.**).
'books' becomes 'nt' (New Testament).
'nut'+'rime'+'nt'='NUTRIMENT'
Can you help me to learn more?
(Other definitions for nutriment that I've seen before include "Material providing food" , "It sustains" , "Nourishing food" , "diet" , "Sustenance, nourishment" .)