Son lost tough fight (4)
I believe the answer is:
tiff
'fight' is the definition.
(thesaurus)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for tiff that I've seen before include "A spat or small row" , "causing quarrel" , "Quarrel (colloq.)" , "(Lovers') quarrel" , "Petty argument" .)