Design a tailless aircraft (4)
I believe the answer is:
plan
'design' is the definition.
('plan' can be a synonym of 'design')
'a tailless aircraft' is the wordplay.
'a tailless' means to remove the last letter.
'aircraft' becomes 'plane' (plane is a kind of aircraft).
'plane' with its last letter taken off is 'PLAN'.
(Other definitions for plan that I've seen before include "Agenda" , "Design; map" , "Stratagem" , "propose" , "Scale drawing; scheme" .)