Difficult son spat (5)
I believe the answer is:
stiff
'difficult' is the definition.
(I've seen this before)
'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 "cheat" , "One dead" , "Rigid, unbending" , "tough" , "Formal" .)