I’m a good source of Expressionist picture (5)
I believe the answer is:
image
'picture' is the definition.
('image' can be a synonym of 'picture')
'i'm a good source of expressionist' is the wordplay.
'a good' becomes 'ag' (I've seen this in other clues).
'source of' says to take the initial letters.
The initial letter of 'expressionist' is 'e'.
'im'+'ag'+'e'='IMAGE'
(Other definitions for image that I've seen before include "Likeness, might be spitting" , "Representation of person or thing" , "Mental picture; idol" , "personality" , "Public impression" .)