Angry, endless argument causing pain in the heart (6)
I believe the answer is:
sorrow
'pain in the heart' is the definition.
(sorrow is emotional distress)
'angry endless argument' is the wordplay.
'angry' becomes 'sore' (similar in meaning).
'endless' means to remove the last letter.
'argument' becomes 'row' (to row is to argue).
'sore' with its last letter taken off is 'sor'.
'sor'+'row'='SORROW'
'causing' acts as a link.
(Other definitions for sorrow that I've seen before include "Sadness, feeling of regret" , "Sadness,regret" , "'Distress, sadness (6)'" , "Misery" , "Woe" .)