Anger begins in relationships - love ends (3)
I believe the answer is:
ire
'anger' is the definition.
(ire is intense rage)
'begins in relationships love ends' is the wordplay.
'begins' indicates taking the first letters.
'ends' says to take the final letters.
The initial letters of 'in relationships' is 'ir'.
The final letter of 'love' is 'e'.
'ir'+'e'='IRE'
(Other definitions for ire that I've seen before include "passion" , "'Anger, wrath (3)'" , "Paddy" , "Anger, rage" , "exasperation" .)