Describe old currency (7)
I believe the answer is:
expound
'describe' is the definition.
(thesaurus)
'old currency' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'currency' becomes 'pound' (as in pound sterling, British currency).
'ex'+'pound'='EXPOUND'
(Other definitions for expound that I've seen before include "Elucidate" , "Present and explain (a theory)" , "Put forward detailed argument" , "Explain and interpret" , "Elaborate" .)