Greece impounds top European aircraft (6)
I believe the answer is:
glider
'aircraft' is the definition.
(glider is a kind of aircraft)
'greece impounds top european' is the wordplay.
'greece' becomes 'Gr' (abbreviation).
'impounds' indicates putting letters inside.
'top' becomes 'lid' (lid is a kind of top).
'european' becomes 'E' (abbreviation e.g. EU).
'lid'+'e'='lide'
'gr' placed around 'lide' is 'GLIDER'.
(Other definitions for glider that I've seen before include "A sort of possum" , "taking flight?" , "Aircraft designed to fly without an engine" , "Plane without an engine" , "Aircraft supported solely by air" .)