Knocker on small money box (7)
I believe the answer is:
scoffer
'knocker on small money box' is the definition.
The answer and definition can be both people as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'knocker on small money' is the wordplay.
'knocker' becomes 'offer' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'on' means one lot of letters go next to another.
'small' becomes 's' (abbreviation - e.g. clothes size).
'money' becomes 'c'.
's'+'c'='sc'
'offer' put after 'sc' is 'SCOFFER'.
Can you help me to learn more?
(Other definitions for scoffer that I've seen before include "Scornfully derisive person" , "I regard derisively" , "One who pokes fun" , "he eats very quickly" , "I doubt" .)