Artist's conflict with Picasso surreal ultimately (6)
I believe the answer is:
warhol
'artist's' is the definition.
(Andy Warhol)
'conflict with picasso surreal ultimately' is the wordplay.
'conflict' becomes 'war' (war is a kind of conflict).
'ultimately' suggests the final letters.
The final letters of 'with picasso surreal' are 'hol'.
'war'+'hol'='WARHOL'
(Other definitions for warhol that I've seen before include "Andy ......., pop art icon of the 1960s" , "Leader of the Pop Art movement" , "Andy -; he said 'In the future everyone will be famous for fifteen minutes'" , "US exponent of pop art, d. 1987" , "15-minute artist?" .)