He painted his mum, a referee at times (8)
I believe the answer is:
whistler
This clue is a double definition.
'he painted his mum' is the first definition.
(James McNeill Whistler)
'a referee at times' is the second definition.
(I've seen this in another clue)
(Other definitions for whistler that I've seen before include "James --, US artist" , "American artist who painted his mother, d. 1903" , "Siffleur - US painter, d. 1903" , "old kettle, say" , "pretending to be brave in the dark?" .)