Run up in the fashion of the fashion business (3,5)
I believe the answer is:
rag trade
'the fashion business' is the definition.
(I've seen this before)
'run up in the fashion' is the wordplay.
'run' becomes 'dart' (I've seen this in other clues).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'in the' is an insertion indicator.
'fashion' becomes 'rage' (something all the rage is fashionable).
'dart' backwards is 'trad'.
'trad' placed into 'rage' is 'RAG TRADE'.
'of' is the link.
(Other definitions for rag trade that I've seen before include "dressmaking competition" , "Slang for the garment industry" , "newsagent's business?" , "National winner" , "Informal description of clothing industry" .)