Invoice for one working - a large amount (7)
I believe the answer is:
billion
'a large amount' is the definition.
'billion' can be an answer for 'amount' (billion is a kind of amount). I'm not sure about the 'a large' bit.
'invoice for one working' is the wordplay.
'invoice' becomes 'bill' (synonyms).
'for' says to put letters next to each other.
'one' becomes 'I' (Roman numeral).
'working' becomes 'on' (on can mean working or activated).
'bill'+'i'+'on'='BILLION'
Can you help me to learn more?
(Other definitions for billion that I've seen before include "A large number" , "One thousand million" , "1,000,000,000" , "One followed by nine zeros" .)