What the payments would be, on a hundred boxes (5)
I believe the answer is:
rates
'what the payments would' is the definition.
I can't tell whether this definition defines the answer.
'be on a hundred boxes' is the wordplay.
'be' becomes 'rate' (rating is a kind of being).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'a hundred boxes' becomes 's' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'rate'+'s'='RATES'
Can you help me to learn more?
(Other definitions for rates that I've seen before include "classes" , "Tax paid by businesses to a local authority" , "List of charges" , "council tax" , "(Old) property taxes" .)