Drawing of a tree that's unfinished (4)
I believe the answer is:
plan
'drawing' is the definition.
(plan is a kind of drawing)
'tree that's unfinished' is the wordplay.
'tree' becomes 'plane tree' (plane is a type of tree).
'that's unfinished' means to remove the last letter.
'plane' with its last letter taken off is 'PLAN'.
'of a' acts as a link.
(Other definitions for plan that I've seen before include "propose" , "Intention or decision about what one is going to do" , "Detailed proposal" , "Scheme - map" , "Schedule" .)