Extreme anger? It’s the fashion (4)
I believe the answer is:
rage
This clue is a double definition.
'extreme anger?' is the first definition.
(rage is intense anger)
'the fashion' is the second definition.
(something all the rage is fashionable)
'it's' is the link.
(Other definitions for rage that I've seen before include "Speak with great anger" , "Fury, passion" , "Latest fashion; high emotion" , "Intense anger" , "Craze, fad" .)