Following day delivery charges (7)
I believe the answer is:
postage
'following day' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'delivery charges' is the wordplay.
'delivery' becomes 'post' (I've seen this before).
'charges' becomes 'age' (I can't explain this - if you can you should believe this answer much more).
'post'+'age'='POSTAGE'
Can you help me to learn more?
(Other definitions for postage that I've seen before include "It could be first-class" , "Delivery cost" , "Cost to send a mail item" , "Mailing cost" , "Charge for 23 Acrosses" .)