The most common and most likely answer to this crossword clue is the 7 letter word PRAETOR
I've seen this crossword clue in The Daily Telegraph
P
R
A
E
T
O
R
Buddy explains!
The straight part
We know that "magistrate once" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case magistrate once = praetor.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does PRAETOR mean?
PRAETOR noun- (history) The title designating a Roman administrative official whose role changed over time:
- (by extension) A high civic or administrative official, especially a chief magistrate or mayor. Sometimes used as a title.
- (translating Italian "pretore") The title of the chief magistrate, the mayor, and/or the podestà in Palermo, in Verona, and in various other parts of 17th- and 18th-century Italy.