Money old regret putting up (4)
I believe the answer is:
euro
'money' is the definition.
(currency of the Eurozone)
'old regret putting up' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'regret' becomes 'rue' (to rue something is to regret it).
'putting up' is a reversal indicator.
'o'+'rue'='orue'
'orue' back-to-front is 'EURO'.
(Other definitions for euro that I've seen before include "EU money" , "French currency" , "jumper" , "Currency since 2002" , "Finnish currency" .)