Portrait's subject is a really easy one (6)
I believe the answer is:
sitter
'easy one' is the definition.
(eg an easy shot in football)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for sitter that I've seen before include "One minds the baby" , "Person posing for a portrait" , "easy target" , "Sedentary baby minder?" , "Artist's model - an easy catch" .)