Sort of gear that's all the fashion (4)
I believe the answer is:
rage
'fashion' is the definition.
(something all the rage is fashionable)
'sort of gear that's all' is the wordplay.
'gear' can be anagrammed to 'RAGE'.
Though, I'm unsure how the anagram is indicated.
'the' is the link.
Can you help me to learn more?
(Other definitions for rage that I've seen before include "Anger - fashion" , "Terrible temper" , "Fury - fashion" , "Wrath" , "Cult - passion" .)