Extinct language only Army and RAF used? (4)

I believe the answer is:

norn

'extinct language' is the definition.
(Germanic language once spoken in parts of Scotland)

I cannot understand how the rest of the clue works.

(Other definitions for norn that I've seen before include "Former Orkney/Shetland language" , "Orkney/Shetland tongue" , "Old language of Orkney and Shetland" , "Amundsen's fate" , "Form of Norse formerly spoken in Shetland" .)

I've seen this clue in The Telegraph and The Times.

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