Language welcome in law is incomprehensible (7)
I believe the answer is:
swahili
'language' is the definition.
(Swahili is an example)
'welcome in law is incomprehensible' is the wordplay.
'welcome' becomes 'hi' (I've seen this in other clues).
'in' indicates putting letters inside.
'incomprehensible' is an anagram indicator (an incomprehensible jumbling of the letters).
'law'+'is'='lawis'
'lawis' with letters rearranged gives 'swali'.
'hi' going inside 'swali' is 'SWAHILI'.
(Other definitions for swahili that I've seen before include "Language widely used in East Africa" , "East African language" , "African people" , "Language of Kenya and lingua franca of other African areas" , "Common language of E. Africa" .)