Stubborn son spat (5)
I believe the answer is:
stiff
'stubborn' is the definition.
(thesaurus)
'son spat' is the wordplay.
'son' becomes 's'.
'spat' becomes 'tiff' ('tiff' can be a synonym of 'spat').
's'+'tiff'='STIFF'
(Other definitions for stiff that I've seen before include "Unfriendly" , "Awkward" , "Rigid, prim" , "Inflexible" , "Difficult" .)