Explain old money (7)
I believe the answer is:
expound
'explain' is the definition.
(I know that expound is a more specific form of the action dilate)
'old money' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'money' becomes 'pound' (pound sterling).
'ex'+'pound'='EXPOUND'
(Other definitions for expound that I've seen before include "Set out in a detailed and learned way" , "Explain meaning in detail" , "Elucidate" , "Present and explain (a theory)" , "Spell out" .)