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