Interpret old money (7)
I believe the answer is:
expound
'interpret' is the definition.
(I've seen this before)
'old money' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'money' becomes 'pound' (I've seen this in other clues).
'ex'+'pound'='EXPOUND'
(Other definitions for expound that I've seen before include "Explain and interpret" , "Set out in a detailed and learned way" , "Spell out" , "Explain meaning in detail" , "Elaborate" .)