The language of Monsieur Dumas? (4)
I believe the answer is:
urdu
'the language' is the definition.
(Urdu is an example)
'of monsieur dumas?' is the wordplay.
'of' indicates a hidden word.
'URDU' is hidden in the letters of 'monsieur dumas'.
(Other definitions for urdu that I've seen before include "Pakistani tongue" , "Foreign language" , "Language akin to Hindi" , "Language used in Pakistan, India" , "speech" .)