Genuine girl sent mad (8)
I believe the answer is:
sterling
'genuine' is the definition.
(thesaurus)
'girl sent mad' is the wordplay.
'mad' indicates anagramming the letters.
'girl'+'sent'='girlsent'
'girlsent' with letters rearranged gives 'STERLING'.
(Other definitions for sterling that I've seen before include "Of the highest qualiy" , "Genuine - money" , "UK money" , "first-class quality" , "British currency" .)