Language used by only four Dutchmen (4)
I believe the answer is:
urdu
'language' is the definition.
(Urdu is an example)
'used by only four dutchmen' is the wordplay.
'used by' indicates a hidden word.
'URDU' is hidden in the letters of 'only four dutchmen'.
(Other definitions for urdu that I've seen before include "Indian language" , "Channel to north of Isle of Wight" , "One of official languages of Pakistan" , "Sanskrit language" , "Indic language written in Persian script" .)