Old police, rising before judge, depart (4)
I believe the answer is:
ogpu
'old police' is the definition.
'OGPU' can be an answer for 'police' (I've seen this before). I'm unsure of the 'old' bit.
'rising before judge depart' is the wordplay.
'rising' shows that the letters should be reversed in order (going up in a down clue).
'before judge' becomes 'up' (up in court).
'depart' becomes 'go' (synonyms).
'up'+'go'='upgo'
'upgo' backwards is 'OGPU'.
Can you help me to learn more?
(Other definitions for ogpu that I've seen before include "Soviet secret police, 1923-34" , "Russian secret police nineteen twenty two to nineteen thirty four; go up (anag.)" .)