Occasionally require old currency (4)
I believe the answer is:
euro
'currency' is the definition.
(currency of the Eurozone)
'occasionally require old' is the wordplay.
'occasionally' means one should take alternating letters.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
The alternating letters of 'require' are 'eur'.
'eur'+'o'='EURO'
(Other definitions for euro that I've seen before include "Coin issued Jan first, two thousand and two" , "The Vatican's currency" , "Basic monetary unit for 19 countries" , "Major currency" , "Ready, in many places" .)