Morality of French and this Latin rests on semantics ultimately (6)
I believe the answer is:
ethics
'morality' is the definition.
(synonyms)
'french and this latin rests on semantics ultimately' is the wordplay.
'french and' becomes 'et' ('and' in the French language).
'this latin' becomes 'hic' ('this' in Latin).
'rests on' means one lot of letters go next to another (in a down clue, some letters rest on top of others).
'ultimately' says to take the final letters.
The last letter of 'semantics' is 's'.
'et'+'hic'+'s'='ETHICS'
'of' is the link.
(Other definitions for ethics that I've seen before include "Work of Aristotle" , "Science of morals" , "Science of moral principles" , "Moral principles in middle" , "Code of practice in business ( 6)" .)