Hollow tree in southeast is plane (8)
I believe the answer is:
spitfire
'is plane' is the definition.
(I've seen this before)
'hollow tree in southeast' is the wordplay.
'hollow' becomes 'pit' (pit is a kind of hollow).
'tree' becomes 'fir' (fir is a kind of tree).
'in' indicates putting letters inside.
'southeast' becomes 'SE' (synonyms).
'pit'+'fir'='pitfir'
'pitfir' put into 'se' is 'SPITFIRE'.
(Other definitions for spitfire that I've seen before include "Fighter aircraft in WWII" , "British fighter plane of WW2, or a fiery-tempered person" , "World War Two fighter aircraft" , "RAF WW2 fighter" , "Ill-tempered sort" .)