Even though it's been explained, I still think that it's a poor clue. If REME is supposed to be 'introducing' something, then it should be at the beginning of the answer, not spread around inside it.
Jeez, I'm so stupid. I was taking one as "une" rather than "un", leading me to miss competely REME, who only happened to be the other regiment in Girdwood Barracks when I was in the TA. I really should shoot myself some days.