What does the cryptic crossword clue "Extinct language only Army and RAF used? (4)" mean?

The definition is "Extinct language". The wordplay uses a charade (building blocks). NO (synonym="NO") + RN (synonym="RN") = NORN The answer is NORN.

Extinct language only Army and RAF used? (4)

Times Cryptic #27938 | 25 Across | 2021-03-30 flower