Kind of painter, British, is in fashion (6)
I believe the answer is:
cubist
'kind of painter' is the definition.
'british is in fashion' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'in' means one lot of letters goes inside another.
'fashion' becomes 'cut' (cut is a kind of fashion).
'b'+'is'='bis'
'bis' placed within 'cut' is 'CUBIST'.
(Other definitions for cubist that I've seen before include "kind of artist" , "Artist like Picasso" , "Geometrical painter" , "Maybe multifaceted" , "modern artist" .)