Diagram of tool unfinished (4)
I believe the answer is:
plan
'diagram' is the definition.
(I've seen this in another clue)
'tool unfinished' is the wordplay.
'tool' becomes 'plane' (plane is a kind of tool).
'unfinished' means to remove the last letter.
'plane' with its final letter taken away is 'PLAN'.
'of' is the link.
(Other definitions for plan that I've seen before include "Formulate" , "Stratagem" , "Programme" , "Large-scale map" , "Scale drawing; scheme" .)