One who painted his mother, a referee? (8)
I believe the answer is:
This clue could be a double definition.
'one who painted his mother' is the first definition.
The answer is a person as well as being a singular noun. This is suggested by the definition. I also know that both 'painted' and 'mother' relate to this answer.
'a referee?' is the second definition.
(I've seen this before)
Can you help me to learn more?
(Other definitions for whistler that I've seen before include "Artist who painted his mother in 1871" , "old kettle, say" , "He used canvas" , "American painter, 1834-1903" , "Olympic ski resort" .)