Number one painting? (4-8)
I believe the answer is:
self-portrait
'number one painting?' is the definition.
'self portrait' can be an answer for 'painting?' (I've seen this before). I am not certain of the 'number one' bit.
'number one painting?' is the wordplay.
'number one' becomes 'self'.
'painting?' becomes 'portrait' (portrait is a kind of painting).
'self'+'portrait'='SELF-PORTRAIT'
(Other definitions for self-portrait that I've seen before include "Artist's own picture" , "Picture of its artist" , "Painting of the artist by the artist" .)