Aircraft heads for Greece's largest island, doing extra refuelling (6)
I believe the answer is:
glider
'aircraft' is the definition.
(glider is a kind of aircraft)
'heads for greece's largest island doing extra refuelling' is the wordplay.
'heads for' indicates taking the first letters.
The first letters of 'greeces largest island doing extra refuelling' is 'GLIDER'.
(Other definitions for glider that I've seen before include "Unpowered aircraft" , "Kind of aircraft without an engine" , "Powerless traveller" , "taking flight?" , "Aircraft supported solely by air" .)