Old regret returning money (4)
I believe the answer is:
euro
'money' is the definition.
(currency of the Eurozone)
'old regret returning' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'regret' becomes 'rue' (synonyms).
'returning' is a reversal indicator.
'o'+'rue'='orue'
'orue' backwards is 'EURO'.
(Other definitions for euro that I've seen before include "Spanish currency unit" , "Coin issued Jan first, two thousand and two" , "Modern cross-border currency" , "Irish currency" , "wallaroo" .)