The matter understood, a law on man's language (7)
I believe the answer is:
italian
'language' is the definition.
(Italian is an example)
'the matter understood a law on man's' is the wordplay.
'the matter' becomes 'I' (I is an example).
'understood a law on' becomes 'talia' (I am not sure about this - if you are sure you should believe this answer much more).
'man' becomes 'N' (knight, a man in chess).
'i'+'talia'+'n'='ITALIAN'
Can you help me to learn more?
(Other definitions for italian that I've seen before include "drink" , "A Latin I know from Rome, say" , "Language of modern Rome" , "Nationality of citizen of Capri" , ", I can appear volatile" .)