Unfold old money (7)
I believe the answer is:
expound
'unfold' is the definition.
(thesaurus)
'old money' is the wordplay.
'old' becomes 'ex'.
'money' becomes 'pound' (I've seen this before).
'ex'+'pound'='EXPOUND'
(Other definitions for expound that I've seen before include "Set out in a detailed and learned way" , "Put forward detailed argument" , "Present and explain (a theory)" , "Upend ox (anag)" , "Set forth, explain in detail" .)