Money old bemoan sending back (4)
I believe the answer is:
euro
'money' is the definition.
(currency of the Eurozone)
'old bemoan sending back' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'bemoan' becomes 'rue' (I've seen this in another clue).
'sending back' is a reversal indicator.
'o'+'rue'='orue'
'orue' backwards is 'EURO'.
(Other definitions for euro that I've seen before include "wallaroo" , "The "single currency"" , "Vatican currency" , "EU money" , "New currency" .)