Design detailed aircraft (4)
I believe the answer is:
plan
'design' is the definition.
(I know that design is a kind of designate)
'detailed aircraft' is the wordplay.
'detailed' means to remove the last letter (the word must be end-less or 'de-tailed').
'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 "Formulate" , "Devise a strategy" , "Scale drawing of a structure" , "Layout drawing" , "Scheme to achieve something" .)