Grind away before sealing arrangement with bank (5)
I believe the answer is:
erode
'grind away' is the definition.
(I've seen this in another clue)
'before sealing arrangement with bank' is the wordplay.
'before' becomes 'ere' (poetic term for 'before').
'sealing' indicates putting letters inside (inserted letters are sealed inside).
'arrangement with bank' becomes 'od' (I can't justify this - if you can you should give a lot more credence to this answer).
'ere' placed around 'od' is 'ERODE'.
Can you help me to learn more?
(Other definitions for erode that I've seen before include "go to pieces!" , "fragment" , "Gradually wear away" , "'Deteriorate, wear away (5)'" , "Abrade" .)