Invoice I owe by November - a large amount (7)
I believe the answer is:
billion
'a large amount' is the definition.
(large number)
'invoice i owe by november' is the wordplay.
'invoice' becomes 'bill' (synonyms).
'owe by' becomes 'o' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'november' becomes 'N' (phonetic alphabet: alpha, bravo, charlie etc.).
'bill'+'i'+'o'+'n'='BILLION'
Can you help me to learn more?
(Other definitions for billion that I've seen before include "One thousand million" , "One followed by nine zeros" , "1,000,000,000" , "A large number" .)