Scottish prosecutor, female, is about fifty (6)
I believe the answer is:
fiscal
'scottish prosecutor' is the definition.
(short for procurator fiscal, Scottish legal term)
'female is about fifty' is the wordplay.
'female' becomes 'fisc' (I am not sure about this - if you are sure you should believe this answer much more).
'is about' becomes 'a' (abbreviation in Chambers).
'fifty' becomes 'l' (Roman numeral).
'fisc'+'a'+'l'='FISCAL'
Can you help me to learn more?
(Other definitions for fiscal that I've seen before include "Budgetary" , "Relating to taxation" , "Califs (anag.)" , "Relating to taxes" , "regarding tax" .)