Surrealist friend has internal fortitude (8)
I believe the answer is:
magritte
'surrealist' is the definition.
(Rene Magritte, Belgian artist)
'friend has internal fortitude' is the wordplay.
'friend' becomes 'mate' (mate is a kind of friend).
'has internal' means one lot of letters goes inside another.
'fortitude' becomes 'grit' (grit is a kind of fortitude).
'mate' placed around 'grit' is 'MAGRITTE'.
(Other definitions for magritte that I've seen before include "Belgian surreal painter" , "Belgian surrealist artist" , "Rene -, Surrealist painter" , "Belgian surrealist painter, d.1967" , "RenZ -, Belgian artist" .)