He painted Ruskin's adversary (8)
I believe the answer is:
whistler
'he painted' is the definition.
(I've seen this before)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for whistler that I've seen before include "He used canvas" , "English painter or mouth musician" , "Artist who painted his mother in 1871" , "American artist who painted his mother, d. 1903" , "He famously painted his mother" .)