Reckoning no one will turn over large amount (7)
I believe the answer is:
billion
'large amount' is the definition.
(large number)
'reckoning no one will turn' is the wordplay.
'reckoning' becomes 'bill' (I've seen this before).
'one' becomes 'i' (Roman numeral).
'will turn' says the letters should be written in reverse.
'no'+'i'='noi'
'noi' written backwards gives 'ion'.
'bill'+'ion'='BILLION'
'over' acts as a link.
(Other definitions for billion that I've seen before include "1,000,000,000" , "One followed by nine zeros" , "A large number" , "One thousand million" .)