Step an artist can never quite finish? (5)
I believe the answer is:
stair
'step' is the definition.
(a single step in a staircase)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for stair that I've seen before include "Rising step" , "that's between levels" , "this could get one down" , "Step as part of flight" , "section of flight" .)