Fight and knock son over (4)
I believe the answer is:
spat
'fight' is the definition.
(thesaurus)
'knock son over' is the wordplay.
'knock' becomes 'tap' (synonyms).
'son' becomes 's' (genealogical abbreviation for son).
'over' says the letters should be written backwards.
'tap'+'s'='taps'
'taps' backwards is 'SPAT'.
'and' is the link.
(Other definitions for spat that I've seen before include "Slight quarrel - ejected saliva" , "Minor argument" , "Small quarrel between Pats" , "Expelled" , "Trivial quarrel" .)