Quarrel that can be relatively bitter (4)
I believe the answer is:
feud
'quarrel' is the definition.
(I've seen this in another clue)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for feud that I've seen before include "State of prolonged mutual hostility" , "Long-running disagreement" , "Prolonged quarrel" , "Bitter quarrel between two parties" , "Private war" .)