Argued violently about foolish person’s appreciation of kindness (9)
I believe the answer is:
gratitude
'appreciation of kindness' is the definition.
(I've seen this before)
'argued violently about foolish person's' is the wordplay.
'violently' indicates anagramming the letters.
'about' means one lot of letters goes inside another.
'foolish person' becomes 'tit' (informal term for a stupid person).
'argued' is an anagram of 'graude'.
'graude' placed around 'tit' is 'GRATITUDE'.
(Other definitions for gratitude that I've seen before include "Feeling of appreciation" , "Appreciation of kindness" , "A sense of indebtedness and thankfulness" , "Thankfulness and appreciation" , "Thanks" .)