First-class police force once set up court in Rome (5)
I believe the answer is:
curia
'court in rome' is the definition.
'curia' can be an answer for 'court' (type of court in ancient Rome). I am not sure about the 'in rome' bit.
'first-class police force once set up' is the wordplay.
'first class' becomes 'ai' (A1 - top quality).
'police force once' becomes 'RUC' (Royal Ulster Constabulary).
'set up' says the letters should be written backwards (in a down clue, letters go up).
'ai'+'ruc'='airuc'
'airuc' in reverse letter order is 'CURIA'.
Can you help me to learn more?
(Other definitions for curia that I've seen before include "Ecclesiastical courts" , "Court of the papal sea" , "Papal court, Vatican government" , "church court" , "Court of the papal see" .)