Lost backing and received double-digit sum (9)
I believe the answer is:
forgotten
'lost' is the definition.
(similar in meaning)
'backing and received double-digit sum' is the wordplay.
'backing' becomes 'for' (I've seen this in another clue).
'and' says to put letters next to each other.
'received double digit sum' becomes 'gotten' (I can't explain this - if you can you should believe this answer much more).
'for'+'gotten'='FORGOTTEN'
Can you help me to learn more?
(Other definitions for forgotten that I've seen before include "Ignored" , "left behind" , "Overlooked" , "Gone from memory" , "Disregarded" .)