Presumably he was happy while painting (8)
I believe the answer is:
whistler
'presumably he was happy while painting' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'presumably he' is the wordplay.
I cannot really understand how this works, but
'he' could be 'hi' and 'hi' is located in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for whistler that I've seen before include "Siffleur - US painter, d. 1903" , "pretending to be brave in the dark?" , "James ---, Rex ---, artists" , "American-born Victorian artist" , "referee" .)