His law changed one language (7)
I believe the answer is:
swahili
'language' is the definition.
(Swahili is an example)
'his law changed one' is the wordplay.
'changed' is an anagram indicator.
'one' becomes 'i' (Roman numeral).
'his'+'law'='hislaw'
'hislaw' anagrammed gives 'swahil'.
'swahil'+'i'='SWAHILI'
(Other definitions for swahili that I've seen before include "Language widespread in East Africa" , "Language of Kenya and lingua franca of other African areas" , "African tongue" , "An official language spoken in 15" , "Major Bantu language widely used in E Africa" .)