Goods about to be put in box (7)
I believe the answer is:
freight
'goods' is the definition.
(freight is a kind of goods)
'about to be put in box' is the wordplay.
'about to' becomes 're' (I can't justify this - if you can you should believe this answer much more).
'be put in' indicates putting letters inside.
'box' becomes 'fight' (I've seen this before).
're' inserted into 'fight' is 'FREIGHT'.
Can you help me to learn more?
(Other definitions for freight that I've seen before include "Charge for carrying" , "Consignment" , "Goods in transit" , "Commercial transport" , "Transport of bulk goods" .)