Painter using black is in fashion (6)
I believe the answer is:
cubist
'painter' is the definition.
(cubist is a kind of painter)
'black is in fashion' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'in' is an insertion indicator.
'fashion' becomes 'cut' (cut is a kind of fashion**).
'b'+'is'='bis'
'bis' going inside 'cut' is 'CUBIST'.
'using' acts as a link.
(Other definitions for cubist that I've seen before include "Picasso was one, for instance" , "kind of artist" , "Painter in a style using geometric shapes" , "Painter like Picasso" , "modern artist" .)