Dress that is, at the time, the rage (6)
I believe the answer is:
attire
'dress' is the definition.
('attire' can be a synonym of 'dress')
'at the time the rage' is the wordplay.
'the time' becomes 't'.
'the rage' becomes 'ire' (I've seen this before).
'at'+'t'+'ire'='ATTIRE'
'that is' acts as a link.
(Other definitions for attire that I've seen before include "Formal clothes" , "Wear" , "Special clothing" , "Habit" , "Clothing, apparel" .)