Excellent book in mostly expensive binding offering harsh commentary (7)
I believe the answer is:
acerbic
'expensive binding offering harsh commentary' is the definition.
I can't tell whether this definition defines the answer.
'excellent book in mostly' is the wordplay.
'excellent' becomes 'ace' (ace can informally mean very good).
'book' becomes 'b' (abbreviation for book).
'in' means one lot of letters goes inside another.
'mostly' becomes 'ric' (I can't justify this - if you can you should believe this answer much more).
'b' placed within 'ric' is 'rbic'.
'ace'+'rbic'='ACERBIC'
Can you help me to learn more?
(Other definitions for acerbic that I've seen before include "Sour; sharp" , "Sour in taste or harsh in tone" , "Bitter, sour" , "Cutting" , "Biting" .)