Tongue and heart of dour duke (4)
I believe the answer is:
urdu
'tongue' is the definition.
(Urdu is an example)
'heart of dour duke' is the wordplay.
'heart of' indicates the answer is hidden within the clue.
'URDU' is hidden within 'dour duke'.
'and' is the link.
(Other definitions for urdu that I've seen before include "Asian language written in Arabic script" , "Language closely related to Hindi" , "Indian language" , "An official language of Pakistan related to Hindi" , "Form of Hindustani" .)