The most common and most likely answer to this crossword clue is the 6 letter word WARDER
I've seen this crossword clue in The Daily Telegraph
W
A
R
D
E
R
More clues leading to the result WARDER
- Officer’s plan again to retreat
- Conflict over the German prison officer
- Fighting the German keeper
- Conflict with the German guard
- I’ll supervise bird being stuffed with central ingredient of mushrooms
- Conflict followed by retreat of revolutionary guard
- Screw, colour green, screwed up
- Conflict with the German prison officer
Buddy explains!

The straight part
We know that "prison officer" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case prison officer = warder.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 WARDER mean?
WARDER noun- A guard, especially in a prison.
- A truncheon or staff carried by a king or commander, used to signal commands.