Our Duncan swallows tongue (4)
I believe the answer is:
urdu
'tongue' is the definition.
(Urdu is an example)
'our duncan swallows' is the wordplay.
'swallows' indicates a hidden word (I've seen 'swallowing' mean this).
'URDU' is hidden in the letters of 'our duncan'.
(Other definitions for urdu that I've seen before include "Channel to north of Isle of Wight" , "Pakistani tongue" , "Sub-continental language" , "Language used in Pakistan, India" , "Sanskrit language" .)