Regret about old currency unit (4)
I believe the answer is:
euro
'currency unit' is the definition.
(currency of the Eurozone)
'regret about old' is the wordplay.
'regret' becomes 'rue' (synonyms).
'about' says the letters should be written backwards.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'rue' reversed gives 'eur'.
'eur'+'o'='EURO'
(Other definitions for euro that I've seen before include "Irish currency" , "The Vatican's currency" , "Currency of Finland" , "wallaroo" , "International currency unit" .)