Indicate one's willingness to remove the top from the moneybox (5)
I believe the answer is:
'indicate one's willingness to remove the top' is the definition.
I can't judge whether this definition defines the answer.
'from the moneybox' is the wordplay.
'from' becomes 'off' (eg someone might be from or 'off' a TV show).
'the moneybox' becomes 'er' (I can't justify this - if you can you should believe this answer much more).
Can you help me to learn more?
(Other definitions for offer that I've seen before include "Tender for acceptance" , "Make available for sale" , "Proposal (of marriage)" , "Suggest" , "Tender, volunteer" .)