Being in favour dulls the tongue (4)
I believe the answer is:
urdu
'tongue' is the definition.
(Urdu is a language or tongue)
'being in favour dulls the' is the wordplay.
'being in' indicates the answer is hidden within the clue.
'URDU' is hidden in the letters of 'favour dulls the'.
(Other definitions for urdu that I've seen before include "Asian language written in Arabic script" , "Pakistani tongue" , "One of official languages of Pakistan" , "Sanskrit language" , "Indian language" .)