Mass in Latin one is ready to give once? (4,3)
I believe the answer is:
lump sum
'ready to give once?' is the definition.
(ready can mean money)
'mass in latin one is' is the wordplay.
'mass' becomes 'lump' (mass can mean a lump or amount of something).
'in latin one is' becomes 'sum' ('I am' or 'one is' in Latin).
'lump'+'sum'='LUMP SUM'
(Other definitions for lump sum that I've seen before include "Large payment, often in cash" , "Single cash payment" , "cash payout" , "Single payment as opposed to many instalments" , "Single payment as opposed to a number of smaller ones" .)