Finish off aircraft design (4)
I believe the answer is:
plan
'design' is the definition.
(I know that design can be written as plan)
'finish off aircraft' is the wordplay.
'finish off' means to remove the last letter.
'aircraft' becomes 'plane' (plane is a kind of aircraft).
'plane' with its last letter taken away is 'PLAN'.
(Other definitions for plan that I've seen before include "Scheme - plot" , "Layout drawing" , "Scale drawing of a structure" , "Aim" , "Programme" .)