Book review discarding book containing unknown bookplate (2,3)
I believe the answer is:
ex lib
'bookplate' is the definition.
I don't know anything about this answer so I can't tell whether this works.
'book review discarding book containing unknown' is the wordplay.
I cannot really understand how this works, but
'book' could be 'b' and 'b' is present in the answer.
'unknown' could be 'x' (algebra) and 'x' is found in the answer.
The remaining letters 'eli' is a valid word which might be clued in a way I don't see.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for ex lib that I've seen before include "From the books of (abbrev.)" , "From the library of" .)