Thus resistance precedes quarrel and sadness (6)
I believe the answer is:
sorrow
'sadness' is the definition.
('sorrow' can be a synonym of 'sadness')
'thus resistance precedes quarrel' is the wordplay.
'thus' becomes 'so' ('so' can be a synonym of 'thus').
'resistance' becomes 'r' (abbreviation).
'precedes' is a charade indicator (letters next to each other).
'quarrel' becomes 'row' ('row' can be a synonym of 'quarrel').
'so'+'r'+'row'='SORROW'
'and' acts as a link.
(Other definitions for sorrow that I've seen before include "Deep sadness" , "'One magpie for ..., two for joy (6)'" , "Sadness,regret" , "Lamenting" , "Woe" .)