First-class material being discussed (2,5)
I believe the answer is:
at issue
'being discussed' is the definition.
(I've seen this before)
'first-class material' is the wordplay.
'first' becomes 'a' (first letter of the alphabet).
'class material' becomes 'tissue' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'a'+'tissue'='AT ISSUE'
Can you help me to learn more?
(Other definitions for at issue that I've seen before include "Questionable" , "under debate" , "In question, under discussion" , "Being discussed or disputed" , "in dispute" .)