Aircraft seen in tree? (5)
I believe the answer is:
plane
This clue is a double definition.
'aircraft' is the first definition.
(plane is a kind of aircraft)
'tree?' is the second definition.
(plane is a type of tree)
'seen in' acts as a link.
(Other definitions for plane that I've seen before include "Tool for smoothing wood" , "Form of transport" , "Nepal (anag.)" , "Smoother" , "Woodworking tool" .)