Magnate from unknown firm in fashion (6)
I believe the answer is:
tycoon
'magnate' is the definition.
(I've seen this before)
'unknown firm in fashion' is the wordplay.
'unknown' becomes 'y' (algebra).
'firm' becomes 'co' (a firm is a company).
'in' means one lot of letters goes inside another.
'fashion' becomes 'ton' (I've seen this before**).
'y'+'co'='yco'
'yco' placed within 'ton' is 'TYCOON'.
'from' is the link.
(Other definitions for tycoon that I've seen before include "Not coy? (anag)" , "One very wealthy" , "Captain of industry" , "Entrepreneur" , "Rich businessperson" .)