Our Duncan's tongue (4)

I believe the answer is:

urdu

I'm a little stuck... Click here to teach me more about this clue!

'tongue' is the definition.
(Urdu is a language or tongue)

'our duncan's' is the wordplay.
'URDU' is hidden in amongst the letters.
But, I am unsure how this is indicated.

Can you help me to learn more?

(Other definitions for urdu that I've seen before include "Language used in Pakistan, India" , "Pakistani language" , "Sanskrit language" , "speech" , "Form of Hindustani" .)

I've seen this clue in The Daily Express and The Mirror.

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