Charge to go up, then down
I believe the answer is:
airfare
'down' is the definition.
I know nothing about this answer so I can't judge whether this works.
'charge to go up then' is the wordplay.
'charge' becomes 'fare' (fare is a kind of charge).
'to' means one lot of letters go next to another (I've seen this in other clues).
'go up then' becomes 'air' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'fare' after 'air' is 'AIRFARE'.
Can you help me to learn more?
(Other definitions for airfare that I've seen before include "Charge for flight" , "The cost of travelling" , "Money for a flight" , "Fee for a flight" , "Cost of a flight" .)