Show bank book (8)
I believe the answer is:
Here is my best explanation:
'book' is the definition.
(I've seen this before)
'show bank' is the wordplay.
'show' becomes 'prove' (proving is a kind of showing).
'bank' becomes 'rbs' (I can't justify this - if you can you should give a lot more credence to this answer).
(Other definitions for proverbs that I've seen before include "Adages, saws", "This book", "Old Testament book ascribed to Solomon, or well-known sayings", "More than one saw", "Book of the Bible", "Saws", "old book", "Truthful expressions", "Gnomes" and "Old Testament Book ascribed to Solomon".)