Top-class police once heading up church court (5)
I believe the answer is:
curia
'church court' is the definition.
(I've seen this before)
'top-class police once heading up' is the wordplay.
'top class' becomes 'AI' (resembles Ar, old-fashioned term meaning excellent).
'police once' becomes 'ruc' (I am not sure about the 'once' bit.).
'heading up' says the letters should be written backwards (in a down clue, letters go up).
'ai'+'ruc'='airuc'
'airuc' back-to-front is 'CURIA'.
Can you help me to learn more?
(Other definitions for curia that I've seen before include "The administrative department of the Vatican" , "church court" , "Court of the papal sea" , "Court of the papal see" , "Papal court, Vatican government" .)