Case of four, then twice that number for cargo (7)
I believe the answer is:
freight
'number for cargo' is the definition.
'freight' can be an answer for 'cargo' (synonyms). I am not sure about the 'number for' bit.
'case of four then twice' is the wordplay.
'case of' suggests removing the centre (outsides of).
'then twice' becomes 'eight' (I can't justify this - if you can you should believe this answer much more).
'four' with its middle removed is 'fr'.
'fr'+'eight'='FREIGHT'
'that' acts as a link.
Can you help me to learn more?
(Other definitions for freight that I've seen before include "Goods carries by large vehicle" , "that's carried by rail and road" , "Cargo, goods transported" , "Goods in transit" , "Something carried" .)